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Genetics, the study of heredity and variation in living organisms, has been a subject of fascination and scientific inquiry for centuries. Understanding how traits are passed from one generation to the next is not only a fundamental aspect of biology but also has significant implications for human health and disease. This report delves into the intricate world of human inheritance, genetic traits, and the prediction of genetic outcomes.
Inheritance involves the transmission of phenotypic traits from one generation to the next through genotypes.
This can be predicted using Punnett squares, which are applicable to both monohybrid and dihybrid traits. Punnett squares provide an expected proportion of phenotypes (Hartley et al., 2016, pp. 474–491).
The monohybrid phenotypic ratio is used to analyze the inheritance of a single characteristic. This ratio spans two generations, F1 and F2. The F1 generation results from crossing two homozygous parents, one homozygous dominant and one homozygous recessive, leading to offspring with a 100% likelihood of carrying the dominant gene and displaying the dominant phenotype.
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All genotypes in the F1 generation contain one dominant allele and one dominant recessive allele (Hartley et al., 2016, pp. 474–491).
The Punnett square for the F1 generation of fly sex inheritance is illustrated below:
| AA | aAa | aAa |
| aAa | Aa | Aa |
| aAa | Aa | Aa |
Key:
The result of this cross is a 100% Aa genotype and 100% wild type phenotype.
The subsequent genetic cross is the F2 generation, which involves crossing the products of the F1 generation.
This results in a 3:1 phenotypic ratio, where 75% of the offspring inherit the dominant allele and display the dominant phenotype. The genotypes for the F2 generation are 1 AA, 2 Aa, and 1 aa (Hartley et al., 2016, pp. 474–491).
The Punnett square for the F2 generation of fly sex inheritance is shown below:
| AA | AAa |
| aAa | aaa |
Key:
This results in a 3:1 phenotypic ratio, with 75% of offspring being wild type and 25% being tetrapter, corresponding to the genotypes 1 AA, 2 Aa, and 1 aa.
| Tetrapter | Wild Type |
|---|---|
| 22 | 78 |
| 22 | 78 |
| 21 | 79 |
| 24 | 76 |
| 25 | 75 |
| 19 | 81 |
| 24 | 76 |
| 28 | 72 |
| 31 | 69 |
| 19 | 81 |
| 25 | 75 |
| 24 | 76 |
| Total = 284 | Total = 916 |
Null Hypothesis (H0): There is no statistically significant difference between the observed frequency of fly types and the expected frequency (3:1).
Alternative Hypothesis: There is a significant difference.
Chi-squared (χ2) test is utilized to determine whether there is a significant difference between the observed (O) and expected (E) frequencies of phenotypic traits.
| Observed (O) | Expected (E) | O-E | (O-E)2 | (O-E)2/E | |
|---|---|---|---|---|---|
| Tetrapter | 284 | 300 | -16 | 256 | 0.853 |
| Wild Type | 916 | 900 | 16 | 256 | 0.284 |
| Σ | 1200 | 1200 | 1.137 |
Degrees of Freedom (D.F.) = n - 1
For this analysis, D.F. = 2 - 1 = 1
The calculated χ2 value is 1.137, which is smaller than the critical value (3.84) for a significance level of 0.05. Therefore, there is no significant difference, and the null hypothesis is accepted. Any observed differences are likely due to chance.
The dihybrid phenotypic ratio pertains to the inheritance of two characteristics simultaneously, controlled by different genes. This analysis involves two generations, the F1 and F2, where the F1 generation comprises two homozygous parents at the two gene loci, resulting in offspring with a 100% chance of a heterozygous genotype and all offspring displaying the dominant phenotype (Hartley et al., 2016, pp. 474–491).
The Punnett square for the F1 generation of corn inheritance is as follows:
| PPSS | ppss |
| ppSS | PpSs |
| ppSS | PpSs |
| ppSS | PpSs |
| ppSS | PpSs |
| ppSS | PpSs |
Key:
The F2 generation results from crossing the offspring of the F1 generation, leading to a 9:3:3:1 phenotypic ratio and nine genotypes. In the example provided, there is a 56.25% chance of the purple smooth phenotype, an 18.75% chance of both the purple wrinkled and yellow smooth phenotypes, and a 6.25% chance of the yellow wrinkled phenotype (Hartley et al., 2016, pp. 474–491).
The Punnett square for the F2 generation of corn inheritance is shown below:
| PS | PpSs | pSps | ppSs | ppss |
| PS | PPSS | PPSs | PpSS | PpSs |
| pSps | PPSs | PPss | PpSs | Ppss |
| ppSs | PpSS | PpSs | ppSS | ppSs |
| ppss | PpSs | Ppss | ppSs | ppss |
Key:
Class data for the dihybrid phenotypic ratio is presented below:
| Purple Smooth | Purple Wrinkled | Yellow Smooth | Yellow Wrinkled | |
|---|---|---|---|---|
| Observed (O) | 2,000 | 1,081 | 2,311 | 386 |
| Expected (E) | 3,250.125 | 1,083.375 | 1,083.375 | 361.125 |
| O-E | -1,250.125 | -2.375 | 1,227.625 | 24.875 |
| (O-E)2 | 1,562,812.516 | 5.641 | 1,507,063.141 | 618.766 |
| (O-E)2/E | 480.85 | 0.0052 | 1391.08 | 1.71 |
Total observations: Σ = 5,778
Degrees of Freedom (D.F.) = n - 1
For this analysis, D.F. = 4 - 1 = 3
The calculated χ2 value is 1,873.64, which exceeds the critical value (7.82) for a significance level of 0.05. Thus, there is a significant difference, and the null hypothesis is rejected in favor of the alternative hypothesis.
Genetic disorders can result from genetic defects or chromosomal abnormalities. They can be classified into different categories, including single-gene inheritance, multifactorial inheritance, chromosomal abnormalities, and mitochondrial inheritance.
Single-gene inheritance involves a change in the base sequence of DNA, leading to incorrect pairing and the coding of the wrong amino acid. This genetic alteration occurs in all cells and affects the primary structure of proteins, resulting in the formation of non-functional proteins. It is caused by a genetic mutation rather than a disease and can manifest as autosomal recessive or dominant traits, as well as being sex-linked.
Multifactorial inheritance is characterized by mutations in multiple genes influenced by a combination of environmental factors.
Chromosomal abnormalities occur during the creation of gametes.
Mitochondrial inheritance is exclusively inherited from the mother.
Autosomal dominant inheritance involves the transmission of a genetic trait or condition from a parent to a child. In this scenario, the mutated gene responsible for the genetic condition is dominant, and children of the affected parent have a 50% chance of inheriting this gene (National Cancer Institute, 2021).
Breast cancer can be associated with an autosomal dominant gene or may result from random mutations in a person's breast tissue. Breast cancer is characterized by the abnormal multiplication of cells in the breast, forming a tumor. It can affect both men and women, with ductal cancer being the most common type. Lobular cancer, which affects the milk-producing glands, is rarer, especially in men (National Cancer Institute, 2021).
Statistics show that breast cancer is most prevalent in women over the age of 45, with approximately 810 cases occurring in this age group. About 80% of breast cancer cases occur in women over 45, and 43% in women over 65. This translates to a 1 in 168 risk of developing breast cancer between the ages of 40-50 and a 1 in 142 risk between the ages of 50-60. Although the chances of developing breast cancer are lower in those under 40, there is still a risk of 1 in 1,227 (0.4%) (Cafasso, 2019).
In the UK, there are around 55,200 new cases of breast cancer reported each year. In 2017, 390 of these cases were in men, accounting for approximately 0.71% of all cases. This means that 1 in every 1,204 individuals will develop breast cancer in the UK each year (Cancer Research UK, 2015).
Not all individuals who develop breast cancer have familial cases. Some cases arise from genetic mutations during mitosis, but for those that are familial, heterozygous individuals have a 50% chance of passing on the breast cancer gene to their offspring. In contrast, homozygous individuals have a 100% chance of passing on the gene.
Punnett squares for familial breast cancer can be illustrated as follows:
| BB | bb |
| bb | Bb |
| BB | Bb |
Result: 100% have familial breast cancer.
Genotypes: 4 Bb
| Bb | bb |
| bB | Bb |
| bB | Bb |
Result: 50% have familial breast cancer, 50% do not have familial breast cancer.
Genotypes: 2 Bb, 2 bb
Autosomal recessive inheritance involves the transmission of a genetic trait or condition to a child when they inherit a copy of the mutated gene from both parents. Both parents will either be carriers of the mutated gene or one will be affected while the other is a carrier. Offspring of two carriers have a 25% chance of not being affected, a 50% chance of being carriers, and a 25% chance of being affected (National Cancer Institute, 2021).
Tay-Sachs disease is a disorder characterized by progressive central nervous system degeneration, usually fatal in children by the age of 5 with no current cure. Treatment focuses on making the patient's life more comfortable. It leads to the accumulation of a fatty substance on nerve cells in the body and becomes apparent months after birth, despite starting during pregnancy (Rochester.edu, 2021). While it is a rare condition, it was most commonly found in the Ashkenazi Jewish population, a significant segment of the UK's Jewish community, but has also been observed in people from other ethnic groups. Symptoms typically manifest at 3-6 months of age and include extreme sensitivity to noise and movement, delayed developmental milestones, muscle weakness progressing to paralysis, difficulty swallowing, vision and hearing loss, muscle stiffness, and seizures. Rarely, Tay-Sachs disease may have later-onset forms, with juvenile Tay-Sachs affecting children and potentially leading to life-threatening complications by the age of 15, and late-onset Tay-Sachs emerging in early adulthood without always resulting in a shortened life expectancy (NHS Choices, 2021).
Tay-Sachs disease is most prevalent in the Ashkenazi Jewish population, with 13,050 babies born to Ashkenazi Jewish couples being affected by the disease, as 130 Ashkenazi Jews are carriers. In the general population, there are approximately 1 in 320,000 babies born with Tay-Sachs disease, with 1 in 300 individuals being carriers. Other groups with a higher likelihood of having children with Tay-Sachs include French-Canadian, Pennsylvania Dutch, and Cajun families (Healthofchildren.com, 2014).
Individuals with infantile or juvenile Tay-Sachs disease are unable to reproduce as they typically do not survive long enough, but those with late-onset Tay-Sachs disease may have offspring who inherit the condition, as symptoms can develop anywhere between the ages of 20 and 80 (NORD - National Organization for Rare Disorders, 2021).
When two carrier parents have offspring:
| TT | tt |
| tt | Tt |
| TT | Tt |
Result: 1 not affected, 2 carriers, 1 affected.
Genotypes: 1 TT, 2 Tt, 1 tt
This produces a 1:2:1 ratio of not affected : carriers : affected.
When one late-onset Tay-Sachs disease parent and one carrier parent have offspring:
| Tt | tt |
| TT | Tt |
| tt | tt |
Result: 2 carriers, 2 affected.
Genotypes: 2 Tt, 2 tt
This produces a 1:1 ratio of carrier : affected.
Dominant and recessive inheritance patterns yield different probabilities. Autosomal dominant disorders have a higher likelihood of inheritance, with a 100% chance for individuals with the dominant gene, a 75% chance for carriers, and a 50% chance for inheritance of the disorder. In contrast, autosomal recessive disorders have a lower likelihood of inheritance as two recessive genes are required for the condition to manifest, resulting in probabilities of 50%, 25%, or 0% for offspring inheriting the disorder (National Cancer Institute, 2021).
Sex-linked inheritance involves the inheritance of a trait through genes located on the X chromosome. This occurs because the X chromosome is larger and carries more genes. In cases of sex-linked inheritance, males are more affected as females possess a healthy X chromosome that can "mask" the effects of the affected X chromosome. In contrast, males have only one X chromosome and are more susceptible to the expression of sex-linked traits (Genome.gov, 2021).
Duchenne Muscular Dystrophy is a neuromuscular disorder that is fatal and follows a sex-linked recessive inheritance pattern, making it more prevalent in males. It results in alterations to the protein dystrophin, leading to muscle degeneration and weakness. Symptoms typically manifest in early childhood, usually around 2-3 years of age. Initial symptoms primarily include muscle weakness, which typically starts in distal limb muscles and progresses to affect the lower extremities, upper extremities, and eventually the heart and respiratory muscles. When it affects the heart and respiratory muscles, it can lead to acute respiratory failure and become fatal. Until recently, individuals affected by Duchenne Muscular Dystrophy did not survive beyond their teenage years. However, advances in cardiac and respiratory care have extended the life expectancy of those with the condition into their 30s (Muscular Dystrophy Association, 2017).
Duchenne Muscular Dystrophy is most commonly found in individuals of European or North American descent and affects approximately 1 in 6,000 males. In the UK, approximately 100 boys are born with the condition each year, with around 2,500 individuals living with the condition at any given time, equating to one in every 26,336 individuals (NHS Choices, 2021).
When a male affected with Duchenne Muscular Dystrophy mates with a female carrier:
| XDY | XdXd |
| XdY | XDXd |
| XDXD | XDXD |
Result: 1 female carrier, 1 female affected, 1 male not affected, 1 male affected.
Genotypes: 1 XDXd, 1 XdXd, 1 XDY, 1 XdY
This produces a 1:1:1:1 ratio of female carrier : female affected : male not affected : male affected.
When a normal male mates with a female carrier:
| XDY | XdXd |
| XdY | XDXd |
| XDXD | XDXD |
Result: 1 female not affected, 1 female carrier, 1 male not affected, 1 male affected.
Genotypes: 1 XDXD, 1 XDXd, 1 XDY, 1 XdY
This produces a 1:1:1:1 ratio of female not affected : female carrier : male not affected : male affected.
A parent with homozygous familial breast cancer and a healthy parent: In this scenario, there is a 100% chance of the offspring developing familial breast cancer. This certainty arises because all offspring inherit one dominant allele for familial breast cancer (BB) from the homozygous affected parent's gametes. Simultaneously, they inherit a recessive allele (aa) from the healthy parent's gametes, ensuring that the gametes from the affected parent will always be dominant and those from the healthy parent will always be recessive. Consequently, there is a 100% chance that the offspring will possess the genome Aa, resulting in the development of familial breast cancer due to the dominant gene.
A parent with heterozygous familial breast cancer (Bb) and a healthy parent (bb): This combination results in offspring having a 1:1 ratio of having familial breast cancer and not having familial breast cancer. The 1:1 ratio occurs because the offspring will invariably inherit one recessive gene (b) from the healthy parent, who only produces recessive gametes. However, the parent with heterozygous familial breast cancer produces a dominant gamete (B) and a recessive gamete (b), meaning that the offspring have an equal chance of inheriting a dominant (B) or recessive (b) gene from the affected parent. Consequently, there is a 50% chance of the offspring having the genotype Bb and developing familial breast cancer, and a 50% chance of having the genotype bb and not developing familial breast cancer.
Two carrier parents (Tt): When both parents are carriers with the genotype Tt, they each produce both dominant (T) and recessive (t) gametes. This results in three possible combinations of genotypes for their offspring. There is a 25% chance of the offspring having the TT genotype, in which they inherit two dominant alleles and are not affected. Additionally, there is a 50% chance of the offspring having the Tt genotype, making them carriers as they inherit a dominant allele from one parent and a recessive allele from the other parent. Lastly, there is a 25% chance of the offspring inheriting two recessive alleles (tt) and being affected by the disorder.
One late-onset Tay-Sachs disease parent (tt) and one carrier parent (Tt): In this scenario, a late-onset Tay-Sachs affected parent with the genotype tt will produce two "t" gametes, ensuring that all their offspring inherit one recessive allele. The carrier parent (Tt) produces one dominant (T) gamete and one recessive (t) gamete, resulting in offspring having a 50% chance of inheriting the dominant allele and a 50% chance of inheriting the recessive allele. Since there is an equal likelihood of inheriting a dominant allele from the carrier parent, the offspring's genotypic ratio is 1:1, with an equal chance of being a carrier or being affected by Tay-Sachs disease.
A Male affected with a female carrier: Duchenne muscular dystrophy is a recessive sex-linked disorder. The affected male has a recessive Duchenne muscular dystrophy allele on the X chromosome, forming the genotype XdY. The male will produce an Xd gamete and a Y gamete. The female carrier has the genotype XDXd and will produce the gametes XD and Xd. Consequently, there are four possible combinations of offspring genotypes. Since the male has a Y chromosome, there is a 50% chance of the offspring also having a Y chromosome. The Xd gamete from the father implies that 50% of the offspring will carry this gene. The female's gametes XD and Xd mean that 50% of the offspring will inherit an XD allele from the female, and the other 50% will inherit the Xd gene. This results in a 25% chance of a female carrier, a 25% chance of a female affected, a 25% chance of a male unaffected, and a 25% chance of a male affected.
Normal male with female carrier: The male's genotype XDY produces the gametes XD and Y, resulting in a 50% chance of the offspring having the XD allele and a 50% chance of inheriting the Y chromosome. The female, with the genotype XDXd, produces the gametes XD and Xd. This produces a 50% chance of the child inheriting the XD gene and a 50% chance of inheriting the Xd gene. The results are a 25% chance of the genotype XDXD (female not affected), a 25% chance of XDXd (female carrier), a 25% chance of the genotype XDY (male not affected), and a 25% chance of the genotype XdY (male affected) offspring.
In conclusion, this report provided a comprehensive overview of human inheritance, genetic traits, and associated disorders. Through the examination of monohybrid and dihybrid phenotypic ratios, Chi-squared analysis, and the discussion of genetic disorders, we gained insights into the principles of classic genetics and the complexities of human genetics.
Our analysis of autosomal dominant, autosomal recessive, and sex-linked inheritance patterns emphasized the importance of genetic counseling, carrier screening, and early diagnosis in managing and preventing genetic disorders. The predictive power of Punnett squares and statistical analysis allowed us to assess the likelihood of specific trait combinations and the accuracy of our genetic predictions.
Ultimately, this knowledge is invaluable in both clinical genetics and genetic research, as it enables us to better understand the inheritance of traits and the risks associated with genetic disorders. As our understanding of genetics continues to advance, we can apply these principles to improve healthcare, develop targeted therapies, and enhance our ability to predict and manage genetic traits and diseases.
Human Inheritance and Predicting Genetic Traits. (2024, Jan 23). Retrieved from https://studymoose.com/document/human-inheritance-and-predicting-genetic-traits
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