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Lecture 5

BIOLOGY 1A03 Lecture Notes - Lecture 5: Dystrophin, Meiosis, Homologous Chromosome

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Rosa Da Silva

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Unit 5 Module 3 Sex Chromosomes and Linkage
Unit 1: The Inheritance of Genes on the X Chromosome
Sex Chromosome
- The chromosomal basis for determining sex is rather unique.
- In humans and other mammals, there are two types of sex chromosomes- the X and Y
chromosome, with the Y chromosome being much smaller than the X chromosome.
- A female will carry two X chromosomes
- a male will develop from a gamete that carries both an X and Y chromosome.
- Most of the regions on the X and Y chromosomes are largely nonhomologous, meaning almost
none of the genes in the X chromosome have counterparts in the Y chromosome,
- recombination between these two chromosomes will not occur
- Sequencing of the human Y chromosome has revealed that there are 78 genes on this
chromosome that code for about 25 proteins (and half of these genes help determine sex).
- In contrast, most of the 1100 genes on the X chromosome have many functions that are
unrelated to sex determination
Inheritance of Sex Chromosomes
- As we follow the inheritance of the sex chromosomes alone, we can see that genes on these
chromosomes contribute to the determination of sex
- In humans, the presence of a set of genes on the Y-chromosome sets off a cascade of events
that leads to the development of the male phenotype.
- There is a 50% chance of a child being male (inheriting the Y chromosome from the father) and a
50% chance of being female (instead inheriting the X chromosome from the father)
- Mendel did not observe this in his pea plants since peas and other plants do not have sex
X Linked Inheritance of Color blindness
- We can follow the inheritance of alleles that lie on the X-chromosome in humans through the
construction of human pedigrees.
- Here we are following the recessive disorder red/green colour-blindness that is inherited as an
X-linked trait that is associated with an X-linked gene on the X chromosome.
- Since this X-linked trait is recessive, this means that women heterozygous for the colour-
blindness allele will not show the phenotype and as a result only women that are homozygous
for the colourblindness allele will be colour-blind.
- However, a heterozygous mother for the colour-blindness allele is a carrier for the allele on
one of her X chromosomes and can certainly pass on this allele to her offspring
- Any male that receives the recessive colour-blindness allele from his mother will be colour-
- This is because the male will only have one locus for this allele (since the males have only one X
chromosome). This is known as a hemizygous individual, rather than the situations in which
there are two alleles such as heterozygous or homozygous individuals. In the hemizygous
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state, the rule of dominance and recessiveness no longer apply. A male will show the
phenotype associated with the one allele that they carry on their X chromosome.
Segregation of Alleles on X-Chromosomes
X-Linked Inheritance of Hemophilia
- In pedigrees, it is possible to trace females that do not show phenotypes, but are carriers
across many generations. They are heterozygous for the recessive allele and can pass it to
their sons with a 50% probability
- The blood-clotting disorder, haemophilia, is an X-linked recessive trait that results from a
mutation in a gene that encodes for a necessary protein that is required for blood clotting
Unit 2: Genetic Linkage
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