gentics punntte question

Question

Colour-blindness is inherited as a sex-linked recessive trait. Eye colour is an autosomal(non-sex linked) trait in which brown eye colour is dominant to blue. A heterozygous brown eyed, colour-blind male is married to a blue eyed female who is a carrier for colour-blindness.

             -  Use a Punnett square to determine the genotype and phenotypes of their possible offspring.

Answer 1

Punnett square problems are not hard to do--once you have them set up. THAT is the hard part. To solve this problem, we need to first remember that the alleles (letters) written on the outside of the Punnett square represent the possible gametes produced by each parent. The types of gametes that each parent can produce is based on their genotypes. So, the steps for solving the problem are: 1. Determine the genotype of each parent; 2. Determine the types of gametes each parent can produce; 3. Write those gametes on the outside of the Punnett square; 4. Complete the Punnett square. I'll go through the steps in order.

 

1. Determine the genotype of each parent. The problem actually tells you this. It states that the male is heterozygous--this means he has one dominant and one recessive allele for eye color. His genotype for eye color is Bb (if B is the dominant allele for brown eyes and b is the recessive allele for blue eyes). The male is also color-blind. We know the allele for colorblindness (cb) is recessive and that this gene is on the X chromosome. In order to be male, he must have one X and one Y chromosome, but only his X chromosome has an allele for normal or colorblind vision. So the male must have this cb allele on his only X chromosome. Thus, his genotype for vision must be X^cb Y.

 

The problem tells you that the female has blue eyes. Because the allele for blue eyes is recessive, she must have the genotype bb for eye color (because if she had one dominant B allele, she would have brown eyes). You are also told that she is a carrier for color-blindness--that means she is heterozygous. Because she is female she has two X chromosomes. She is not color-blind herself so she must have one allele for normal vision: X^CB. But she is a carrier so she must have one allele for color blindness: X^cb. So her genotype for vision is X^CB X^cb.

 

So the parents' genotypes are Bb X^cbY and bb XCB X^cb.

 

2. Determine the types of gametes each parent can make. Remember that when gametes are made, matched chromosomes (homologous chromosomes) with their alleles line up together and then separate from each other (this is Mendel's 1st law). (Even though they are not completely homologous, the X and Y chromosomes line up together.) So at the end the gametes have just one of each gene. In the case of the male, the B and b alleles separate from each other and each gamete will have EITHER a B OR a b allele. And the X chromosome with the cb allele separates from the Y chromosome, so each gamete can have EITHER an X^cb OR a Y chromosome. Because the eye color and color vision genes are on different chromosomes (eye color on an autosomal, or non-sex, chromosome and color vision on the X chromosome), they move independently of each other (this is Mendel's 2nd law). This means that the B allele can be in a gamete with either X^cb or with Y. And the b allele can be in a gamete with either of these also. So the male can make four different types of gametes: B X^cb; B Y; b X^cb; and b Y. 

 

In the case of the female, all of the gametes will have a b allele because she has two of the same alleles for eye color. She has two different alleles for color vision, however. So half of her gametes will have X^CB allele and half will have X^cb. So she can make two different types of gametes: b X^CB and b X^cb. 

 

3. Write the gametes on the outside of the Punnett square. Write the letters representing the four different types of the male's gametes across the top of the Punnett square. Write the letters representing the two different types of the female's gametes down the left side of the Punnet square. Thus your Punnett square should have four columns and two rows.

 

4. Complete the Punnett square. Then continue to fill in the Punnett square. Each of the cells in the square represents a possible genotype for their offspring. For example, the upper left cell would show the genotype Bb X^cb X^CB. This offspring would have the phenotype brown eyes and normal color vision (and would be a female because there are two X chromosomes).

 

When you complete this you should find that the possible offspring could have brown eyes & normal color vision; brown eyes & colorblindness; blue eyes & normal color vision; or blue eyes & colorblindness. (With a probability of 1/4 of each and equal numbers of male & female offspring in each category.)