Sex-linked inheritance involves the genes on the sex chromosomes. Because inheritance patterns differ between females and males, crosses involving sex-linked inheritance use X’s and Y’s and the results of the crosses are given in percentage of female and male offspring with the affected traits, not in ratios. In humans, the sex chromosomes are X and Y. Females have two X chromosomes (XX) and males have one X and one Y (XY). In males, whatever alleles are on the X chromosome are expressed, whether they are dominant or recessive. In females, if the trait is recessive she is called a carrier and does not express the trait (in other words the dominant, normal trait is expressed). In order for her to express the recessive trait, she must be homozygous recessive (X+X+). If the trait is dominant, she will express the trait. Therefore, a female denoted as X+X-is either a carrier (the most common scenario) or expresses the allele if thetrait is dominant. Colorblindness is an X-linked recessive trait. Consider a cross between a male who is colorblind (X+Y) and a female who is not colorblind (X- X-). Cross a male who is not colorblind with a female who is a carrier. C1) Construct a Punnett square. C2) What percentage of male offspring will have normal vision and what percentage will be colorblind? C3) What percentage of female offspring will have normal vision? Be carriers? Be colorblind?
Sex-linked inheritance involves the genes on the sex chromosomes. Because inheritance patterns differ between females and males, crosses involving sex-linked inheritance use X’s and Y’s and the results of the crosses are given in percentage of female and male offspring with the affected traits, not in ratios. In humans, the sex chromosomes are X and Y. Females have two X chromosomes (XX) and males have one X and one Y (XY). In males, whatever alleles are on the X chromosome are expressed, whether they are dominant or recessive. In females, if the trait is recessive she is called a carrier and does not express the trait (in other words the dominant, normal trait is expressed). In order for her to express the recessive trait, she must be homozygous recessive (X+X+). If the trait is dominant, she will express the trait. Therefore, a female denoted as X+X-is either a carrier (the most common scenario) or expresses the allele if thetrait is dominant. Colorblindness is an X-linked recessive trait. Consider a cross between a male who is colorblind (X+Y) and a female who is not colorblind (X- X-).
Cross a male who is not colorblind with a female who is a carrier.
C1) Construct a Punnett square.
C2) What percentage of male offspring will have normal vision and what percentage will be colorblind?
C3) What percentage of female offspring will have normal vision? Be carriers? Be colorblind?
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