Genetics Answer Note #6

Question 1 What controls polygenic traits? Question 9 options: one dominant and one recessive allele many dominant alleles many recessive alleles a recessive allele controlling a dominant allele multiple recessive alleles controlling a dominant allele Answer Polygenic traits refer to characteristics that are controlled by multiple genes, rather than just a single gene. This means that there are often several different genetic variations, or alleles, that contribute to the expression of these traits. So what exactly controls polygenic traits? According to scientific research, polygenic traits are controlled by multiple genes, each with a small effect, as opposed to one dominant and one recessive allele. This means that multiple genes work together to influence the expression of a polygenic trait, rather than a single dominant gene overpowering a recessive one. This concept is known as polygenic inheritance. In terms of dominant and recessive alleles, it is possible for polygenic traits to be controlled by many dominant alleles or many recessive alleles. However, this is not always the case. In some instances, a recessive allele may control a dominant allele, known as epistasis. This means that the expression of the dominant allele is dependent on the presence of the recessive allele. Another factor that can control polygenic traits is gene linkage. Genes that are physically located close to each other on a chromosome are said to be linked. This means that they are more likely to be inherited together, and therefore have a combined effect on the expression of a polygenic trait. Additionally, environmental factors can also play a role in the expression of polygenic traits. These can include diet, exposure to toxins, and even social factors. Question 2 107.How many true-breeding pea plant varieties were selected by Mendel? (1) 7 (2) 36 (3) 14 (4) 24 108.Which of the following represents the genetic makeup of female grasshopper? (1) AA+XY (2) AA-XO (3) AA + XX (4) AA+YY 109.in which of the following Mendelian crosses both dominant and recessive phenotypes will be observed in the progeny? (1) AABB x aabb (2) AaBb aabb (3) AaBb x AABB (4) aabb x aabb 110. Examine the pedigree chart given below. Which of the given disorders is not applicable in above pedigree. (1) Myotonic dystrophy (2) Cystic fibrosis (3) Sickle cell anaemia (4) Haemophilia 111. T.H. Morgan chose as model organism for experimental verification of chromosomal theory of inheritance. (1) Pisum sativum (2) Human (3) Drosophila melanogaster (4) Neurospora Answer 107. How many true-breeding pea plant varieties were selected by Mendel? Mendel selected 7 true-breeding pea plant varieties for his experiments on inheritance. These varieties were selected based on their distinct physical characteristics, such as flower color, shape of the seed, and height. By crossing these varieties, Mendel was able to observe how certain traits are inherited from one generation to the next. 108. Which of the following represents the genetic makeup of female grasshopper? The genetic makeup of a female grasshopper is AA+XX. This means that the female has two copies of the A sex chromosome and two copies of the X sex chromosome. In grasshoppers, females are the homogametic sex, meaning they have two of the same type of sex chromosome. 109. In which of the following Mendelian crosses both dominant and recessive phenotypes will be observed in the progeny? In a cross between AaBb and aabb, both dominant and recessive phenotypes will be observed in the progeny. This is because the A and B genes are dominant, but the a and b genes are recessive. Therefore, offspring can inherit either the dominant or recessive traits from their parents, resulting in a mix of phenotypes. 110. Examine the pedigree chart given below. Which of the given disorders is not applicable in above pedigree. Based on the pedigree chart given below, the disorder that is not applicable in the above pedigree is sickle cell anemia. This is because all individuals in the pedigree show a clear inheritance pattern of an autosomal dominant disorder, while sickle cell anemia is an autosomal recessive disorder. This means that individuals with one copy of the mutated gene (heterozygotes) do not show any symptoms of the disorder. 111. T.H. Morgan chose as model organism for experimental verification of chromosomal theory of inheritance. T.H. Morgan chose the fruit fly, Drosophila melanogaster, as the model organism for experimental verification of the chromosomal theory of inheritance. This was because fruit flies were easy to breed and had a short lifespan, allowing for multiple generations to be observed in a short period of time. Additionally, fruit flies have distinct physical characteristics that can be easily observed and studied, making them a suitable organism for genetics research. Question 3 Ram and Asha were a married couple where Ram had normal vision whereas Asha could not see some colours (colour-blind). They give birth to five children - 3 sons and 2 daughters. Of these, all sons were colour-blind while both daughters had normal vision. 4 (a) Using a reason to support your answer for each, identify if the trait for colour blindness is: (i) recessive or dominant? (ii) linked to the X chromosome or Y chromosome? Answer Introduction: According to the question, Ram and Asha were a married couple where Ram had normal vision while Asha was colour-blind. Upon giving birth to five children, it was observed that all three sons inherited their mother's colour-blindness trait, while both daughters had normal vision. In this answer, we will explore the genetic inheritance pattern of colour blindness and determine whether it is a recessive or dominant trait, as well as whether it is linked to the X or Y chromosome. (i) Is colour blindness a recessive or dominant trait? To understand whether colour blindness is a dominant or recessive trait, we first need to understand the basics of genetics. Each gene has two copies, one inherited from the father and one inherited from the mother. These copies are called alleles, and can either be dominant or recessive. A dominant allele will always be expressed, while a recessive allele will only be expressed if there are two copies present. Based on the information provided, we can assume that the trait for normal vision is dominant, since both daughters have it. This means that Asha, who is colour-blind, must have two recessive alleles for colour blindness. This also explains why all three sons are colour-blind, as they inherited a recessive allele from their mother and a Y chromosome from their father. (ii) Is the trait for colour blindness linked to the X or Y chromosome? Genetic traits can be linked to specific chromosomes, based on their location on the DNA. Generally, traits that are passed down through the X chromosome are called Xlinked traits, while traits that are passed down through the Y chromosome are called Ylinked traits. In the case of colour blindness, it is an X-linked trait and is inherited through the X chromosome. As mentioned earlier, Asha has two recessive alleles for colour blindness. Since women have two X chromosomes, they can be carriers for the trait without being affected by it. However, men only have one X chromosome, which they inherit from their mother, and they do not have another copy to compensate if the trait is present. This is why all three sons inherited the trait for colour blindness. Conclusion: In summary, the trait for colour blindness is a recessive trait that is linked to the X chromosome. It is interesting to note that while Asha is colour-blind, she is not affected by it to the same extent as her sons. This is because she has two X chromosomes, giving her a backup copy of the normal gene, while her sons only have one X chromosome and no backup copy. This also explains why the daughters did not inherit the trait, as they have two X chromosomes and are not affected by the recessive allele. Question 4 Red eyes (R) in fruit flies are dominant over white eyes (r). Find the possible eye colors of the F1 generation for each of the following crosses. rrxRR Answer In order to answer this question accurately, it is important to have a thorough understanding of inheritance patterns in genetics and the principles of Mendelian genetics. The following is a breakdown of the steps that will help to find the possible eye colors of the F1 generation for the given crosses. Step 1: Understand the Basics of Genetics As per Mendelian genetics, each individual has two copies of a gene, termed as alleles, and these alleles determine the physical characteristics of an organism. In the case of fruit flies, eye color is determined by a single gene with two alleles, red eye (R) and white eye (r). Step 2: Identify the Parental Genotypes The given cross, rrxRR, represents a cross between two parents, one homozygous for the white eye allele (rr) and the other homozygous for the red eye allele (RR). Homozygous individuals have two identical alleles for a particular gene. Step 3: Determine the Gametes Produced by Each Parent During sexual reproduction, gametes are formed by a process called meiosis. In fruit flies, each parent will produce four gametes, with each gamete containing only one allele for the eye color gene. Parent 1 (rr): produces four gametes, each carrying the r allele (r) Parent 2 (RR): produces four gametes, each carrying the R allele (R) Step 4: Determine the F1 Offspring Genotypes In the F1 generation, one parent will pass on one of its alleles (r or R) to its offspring, while the other parent will pass on its other allele (r or R). Thus, the F1 offspring will have a genotype of Rr for their eye color gene. Step 5: Determine the Possible Phenotypes of the F1 Offspring The phenotype is the physical expression of an organism's genotype. In the case of fruit flies, the R allele for red eye color is dominant over the r allele for white eye color. This means that the F1 offspring will all have red eyes, as R is the dominant allele. Question 5 Anne and Gilbert were expecting their first child and during a visit to the gynaecologist they both were asked if there were any hereditary conditions in their family. Anne informed the doctor that her father and maternal grandfather were colorblind and so was her husband, Gilbert. (Assume color blindness to be a recessive trait). a. What is the probability that this child will be colorblind? (2 marks) b. How did Gilbert inherit the gene for color blindness? (1 mark) c. If Anne was colorblind as well, then what would be the probability that their child would be colorblind? (2 marks) Answer According to Mendelian genetics, color blindness is a recessive trait that is expressed on the X chromosome. This means that for an individual to exhibit color blindness, they must have two X chromosomes carrying the gene for color blindness, one from each parent. With this in mind, let's examine each part of the question in detail. A. What is the probability that this child will be colorblind? The probability of a child being colorblind in this scenario depends on the genetic makeup of both parents. Based on the information provided, we know that both Anne and Gilbert are colorblind and they each have a father and a maternal grandfather who are also colorblind. This means that all four grandparents are carriers of the gene for color blindness. In order for their child to be colorblind, both Anne and Gilbert must pass on their X chromosome carrying the gene for color blindness to their child. This is known as a homozygous recessive inheritance. Therefore, the probability of their child being colorblind is 1/4 or 25%. To calculate this probability, we can use a Punnett square. In this case, both Anne and Gilbert have the genotype XcXc, where Xc represents the gene for color blindness. When we cross these genotypes in the Punnett square, we get a 1/4 chance of having a child with the genotype XcXc, which would result in color blindness. B. How did Gilbert inherit the gene for color blindness? Since color blindness is a recessive trait, Gilbert inherited the gene from both of his parents. His mother must have been a carrier of the gene, while his father exhibited color blindness. When Gilbert was conceived, he had a 50% chance of inheriting the X chromosome carrying the gene for color blindness from his mother, and a 50% chance of inheriting a normal X chromosome from his father. C. If Anne was colorblind as well, then what would be the probability that their child would be colorblind? If Anne was colorblind, her genotype would also be XcXc. In this case, the probability of their child being colorblind increases to 50%. This is because both parents have the same genotype, XcXc, meaning there is a higher chance of passing on the gene for color blindness to their child.