Download Monohybrid Crosses Involving Sex

SBI3C Genetics
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Monohybrid Crosses Involving Sex-Linked Traits
Autosomal chromosomes:
chromosome#1-22 (all chromosomes other than the sex chromosomes
that determine non-sexual characteristics, like eye colour, freckles, etc.)
Sex chromosomes:
chromosomes #23 (X and Y) that determine the development of sex
organs and of secondary sex characteristics
Sex-linked inheritance:
the inheritance of NON-SEXUAL traits along with sex traits.
(Such traits appear more frequently in one sex)
Sex-linked characteristics:
non-sexual characteristics that are carried on the X chromosome
(the Y chromosome is much smaller, and only carries information about
the sex of the organism)
E.g. hemophilia: a type of bleeding disorder in which blood takes
a long time to form clots (recessive trait)
E.g. colour-blindness: inability to see red/green (recessive trait)
Female
XX
Male
(males determine the sex of the child)
XY
Example 1: Suppose a man who does not have hemophilia marries a woman who is a carrier.
(a) Show the genetic cross.
LEGEND: genotypes and phenotypes for hemophilia
XHXH
XHXh
XhXh
XHY
XhY
♀, non hemophilia (homozygous)
♀, non hemophilia (carrier)
♀, hemophilia
♂, non hemophilia
♂, hemophilia
(b) What is the possibility of the sons having hemophilia?
(c) What is the possibility of the daughters having hemophilia?
(d) What are the chances of them having children without hemophilia?
Example 2: If a man who is not colour-blind (x-linked recessive trait) marries a colour-blind woman, is
there any chance of them having a son who is not colour-blind? Show the cross and explain the answer.
Homework Questions
1. A man who is colour-blind marries a woman who is not colour-blind (and not a carrier either).
(a) What are the chances of them having a son who is colour-blind?
(b) What are the chances of them having a daughter who is colour-blind?
(c) What are the chances of them having a child who is not colour-blind?
2. A woman homozygous for hemophilia marries a man who does not have hemophilia.
(a) What are the chances of them having a son who has hemophilia?
(b) What are the chances of them having a daughter who has hemophilia?
(c) What are the chances of them having kids without hemophilia?
3. A man who is colour-blind marries a woman who is a carrier for colour-blindness.
(a) What are the chances of them having a son who is not colour-blind?
(b) What are the chances of them having a son who is colour-blind?
(c) What are the chances of them having a daughter who is not colour-blind?
(d) What are the chances of them having a daughter who is colour-blind?
4. A hemophiliac male marries a female who is a carrier of hemophilia. What is the probability of the
couple having a male who is a hemophiliac?
5. If a male is colour-blind, which parent is responsible for passing on the trait to him?
6. In order for a female to be colour-blind, do both of her parents have to be colour-blind?