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Lecture 49

HSC 2100 Lecture Notes - Lecture 49: Homologous Chromosome, Haemophilia, Color Blindness

Health Sciences
Course Code
HSC 2100
James Crawlynn

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Crossing Over
Crossing over of genetic material during meiosis results in the exchange of genes between the
homologous chromosome pairs
The combinations of alleles of the gametes will vary across cells and differ from the parent
Two alleles which are not dominant over each other are 'equally dominant'
- Both alleles are expressed in each others presence
-The phenotypic effects of each allele are fully simultaneously expressed in the heterozygote. Eg, black
and white co-dominant alleles for fur coat colour may lead to a black-white speckled cat
Sex in humans is determined by the combination of sex chromosomes received by offspring.
Homozygous XX combinations lead to females and heterozygous XY combinations lead to males.
Certain traits are carried on these sex chromosomes thus inheritance of certain traits may be influenced
by the sex of an individual.
If a trait is carried on the X chromosome, it can appear in both males and females, however it is likely
that a greater proportion of males receive recessive sex-linked traits as they only receive a single X
If a trait is carried on the Y chromosome, only males can receive, express and pass-on the trait.
Examples of sex-linkage
Haemophilia and colour blindness are all genetic traits found on the x chromosome, therefore can occur
in both males and females, however occur in greater proportion of males than females. This is because
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males only receive one copy of the x-chromosome and if these recessive conditions are present they will
always be expressed.
Thomas Morgan
- Thomas Morgan was an American scientist who experimented using the fruit fly 'Drosophila
melanogaster' which has 4 pairs of chromosomes, one of which is a pair of sex chromosomes.
- Morgan's crosses were designed to determine if white eyes were inherited in Mendelian fashion and
so like Mendel, he interbred monohybrid-cross organism.
Morgan's results did not show the expected 3:1 ratio typical of Mendelian inheritance and they showed
a difference depending on whether the gene was inherited from a male or female. Morgan
hypothosised the gene was found on the X chromosome, therefore he introduced the idea of sex-linked
Influence of environment on gene expression
The environment of an organism can impact the extent of gene expression in the phenotype, for
example height capacity may not be reached in humans due to malnutrition, the soil pH may impact the
flower colour of hydrangeas, the consumption of food can impact the feather colouring of flamingoes
DNA Replication steps
1. Double helix strands are unwound and seperated by the helicase enzyme, leaving two nucleotide
2. Free nucleotides floating in the nuclear sap bond with corresponding exposed nucleotides on the
chains due to the enzyme DNA polymerase. This form two DNA chains. Enzyme also 'proofreads' this
chain of nucleotides for ERRORS.
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