X-Linked traits and recombination

• Sex-linked genes involve genes on sex chromosomes (X or Y) and results differentiate between male and female.
• Heterogametic means 2 different types of gametes. Usually male: X, Y chromosome.
• Homogametic means 1 type of gamete. Usually in females, X chromosomes only.
• X-linked recessive are traits controlled by recessive genes on the X chromosome. They are mostly common in males, 1 X chromosome. Females need to inherit 2 copies on the recessive allele.
• X-linked dominant is expressed in both male and female. Males may show different expressitivity and females if they inherit 1 copy of the dominant allele.
• Y-linked inheritance are genes on the Y chromosome are passed on from father to son and not expressed in females. Y chromosome is infertility disorder.
• A father with the specific trait will transmit the mutant allele to all daughters, but no sons. This is because the mother is a dominant homozygote. The daughters will have a normal phenotype, but will have carriers of mutation.
• A carrier who mates with a normal male will pass the mutation to her half sons and half daughters. The half sons will have a mutation and the half daughters will be carriers like the mother.
• A carrier mates with a male with trait, there is a 50% chance that each child will have the trait. Daughters without the trait will be carriers. Males without the trait will be free from all the harmful recessive allele.
• There are 20,000 genes in humans, 23 pairs of chromosomes, (1000 linked genes).
• When genes are close together on a chromosome, the alleles on the same chromosome tend to be inherited, so it show non-independent assortment and are linked.
• We can determine the order and location of genes within a chromosome using recombination.
• Getting a double heterozygote means we know which alleles are together on the chromosome.
• By finding recombinant events, we look at the gametes made by the heterozygous parent.
• Parental types results are obtained by morgan.
• We can combinants from crossing over in meiosis called recombination.
• During crossing over, two chromatids on a gene tetrad cross over at random points and swap genetic material when only 1 chiasmata is formed.
• From getting combinants, we are able to calculate the recombinant frequency.
• RF = recombinants / total offspring x 100
• RF of the gametes contain a chromosome that has formed a chiasma between the 2 genes.
• The smaller the distance of 2 genes, less likely a chiasma will be formed, called unlinked genes.
• Unlinked genes will have a RF of 50%, 1/2 recombinant and 1/2 parental. There is a linear relationship between distance and RF
• Fewer recombinants means the 2 genes must be linked (on the same chromosome).