Sex-linked Genes & Chromosomes lecture 20

Lecture 20 Objectives

• After revision, an understanding of sex-linked gene inheritance, non-independent assortment in linked genes, and recombination via crossing over is expected

Sex-Linked Traits

• Sex-linked traits involve genes located on sex chromosomes (X or Y) instead of autosomes
• The inheritance patterns for sex-linked traits differ between males (XY) and females (XX)
• Examples of sex-linked traits include haemophilia and red-green colour blindness

Sex Determination

• Sex chromosomes (X and Y) determine an individual's sex
• Other chromosomes are called autosomes
• In bees, fertilised eggs develop into diploid females, while unfertilised eggs become haploid males

Heterogametic vs Homogametic Chromosomes

• Male birds carry two identical sex chromosomes (ZZ), and females have two different ones (ZW)
• Drosophila females have two X chromosomes and may also have a Y chromosome; males have one X chromosome, and may have a Y chromosome, and be fertile

Eye Colour as a Sex-Linked Trait in Drosophila

• Eye colour is a sex-linked trait in Drosophila
• When crossing a homozygous red-eyed female with a hemizygous white-eyed male, all daughters are red-eyed heterozygotes and all sons are red-eyed hemizygotes

Red-Green Colour Blindness

• The majority of people with red-green colour deficiencies should read the number 5 when viewing colour blindness test images
• Red-green colour blindness is a sex-linked trait
• When a normal female crosses with a colour-blind male, all daughters are carriers and all sons are normal
• A carrier female crossed with a normal male results in half the daughters being normal, half being carriers, half the sons being normal and half being colour blind

Inheritance Patterns with X-Linked Alleles

• A father expressing an X-linked trait transmits the mutant allele to all daughters but no sons
• If the mother is homozygous dominant, daughters will have a normal phenotype but be carriers
• A carrier female mating with a normal male passes the mutation to half her sons and half her daughters
• Sons with the mutation will have the disorder, and daughters with a single dose will be carriers
• A carrier female mating with a male with the trait has a 50% chance each child inherits the trait
• Daughters not expressing the trait will be carriers, while males without the trait will be free of the harmful recessive allele

Gene Mapping

• There are approximately 20,000 genes in humans
• Humans have 23 pairs of chromosomes
• This equates to about 1,000 linked genes per chromosome
• Linked genes tend to be inherited showing biases towards parental genotypes
• Recombination frequencies allow determination of gene order and location on a chromosome

Conventions for Fruit Fly (Drosophila) Genes

• The first mutant discovered for a gene provides the letters for symbolizing it like 'vg' for vestigial wings
• The wild-type allele is indicated by a "+" after the symbol, e.g., 'vg+' for wild-type wings
• vg is recessive to vg+

Example of a Genetic Cross Involving Linked Genes

• In a cross between a wild-type (b+b vg+vg) and a mutant (bb vgvg) Drosophila, the F1 generation will be heterozygotes (b+b vg+vg)
• Considering linked genes, a 1:1:1:1 phenotypic ratio is anticipated in the offspring of the cross

Outcome of Test Cross

• A test cross occurs when a heterozygote is crossed with a homozygote recessive
• Two genes do not sort independently; they are linked on the same chromosome

Recombination and Parental Genotypes

• Not all gametes have parental genotypes because of parental chromosomes with wild type alleles and homologous chromosomes which are paired and replicated
• Meiosis I continues with meiosis II and the chromosomes all seperate

Recombinants

• Recombinants occur because of crossing over during meiosis

"Crossing Over"

• 'Crossing over' during meiosis explains recombination
• Two chromatids of a tetrad cross over at random points and swap genetic material
• The proportion of recombinant gametes is termed the recombination frequency

Calculating Recombination Frequency

• 17% of gametes contain a chromosome that has formed a chiasma between two genes
• Recombination Frequency = (number of recombinant offspring) / (total number of offspring) = (206+185) / (944+965+206+185) = 0.17 or 17%

Gene Distance and Chiasma Occurence

• A smaller distance between two genes means a chiasma is less likely to form between them
• Distant (unlinked) genes have recombination frequencies around 50%
• Close genes have recombination frequencies between 0-50%
• An almost linear relationship exists between distance and recombination frequency

Genetic Mapping

• Sturtevant created a partial map of Drosophila using recombinant frequencies of five traits
• He used an arbitrary unit of distance, the centimorgan (cM), where 1 cM = 100 x recombination frequency

Example Question: Flowering Plants

• In a cross of PpTt x pptt we see non-mendelian ratios in the offspring
• Purple tall (345), white short (340), white tall (255) and purple short (260)
• Calculate recombination frequency of (255 + 260)/(345 + 340 + 255 + 260) = 515/1200 = 0.429 (42.9%)
• Fewer recombinants means the 2 genes must be linked (on the same chromosome), violating Mendel's 2nd Law (independent assortment)

Lecture 20 Summary

• Sex-linked genes exhibit inheritance patterns varying between sexes
• X-linked recessive traits, such as red-green colour blindness, are more prevalent in males than in females
• Crossing over during meiosis can shuffle genetic material
• The distance between linked genes can be estimated by the proportion of new phenotypes/genotype combinations from a test cross

Objective-Based Questions

• Define carrier, homozygous, recombinant, and hemizygous
• Eye colour is a sex-linked trait in Drosophila, located on the X chromosome
  • a) Consider the sex, genotypes, and phenotypes of offspring from a cross between a homozygous red-eyed female and a hemizygous white-eyed male
  • b) Consider the sex, genotypes and phenotypes of the offspring from a cross between a homozygous white eyed female with a hemizygous red eyed male
• What are linked genes and how are they inherited? Are they biased towards parental or recombinant genotypes?
• Fill in the gap: The lesser the distance between two genes, the more likely a chiasma will be formed between them. Distant genes have higher recombination frequencies, there is a near linear relationship between distance and frequency