Genetics Concepts: Sex-Linked Traits, Inheritance, and Population Genetics

What does it mean for a trait to be sex-linked?

It is determined by genes on sex chromosomes and inherited differently depending on which sex is hemizygous.

What is the purpose of X-inactivation in mammals?

To equalize gene dosage between XX females and XY males by silencing one X chromosome.

What is incomplete dominance?

A heterozygote shows an intermediate phenotype between the two homozygotes.

What is codominance?

Both alleles are fully expressed in the heterozygote.

What is penetrance?

The percentage of individuals with a genotype who actually express the phenotype.

What is expressivity?

The degree to which a trait is expressed by individuals with the genotype.

How do lethal alleles affect ratios?

They eliminate individuals with certain genotypes, producing a 2:1 phenotypic ratio.

What are multiple alleles?

More than two possible alleles exist in a population even though individuals have only two.

What is gene interaction?

When multiple genes influence a single trait.

What is epistasis?

When one gene masks or modifies the expression of another.

How can sex influence autosomal gene expression?

Hormones can alter expression, producing sex-influenced or sex-limited traits.

What is cytoplasmic inheritance?

Inheritance of traits encoded in mitochondrial or chloroplast DNA, usually maternal.

What is the genetic maternal effect?

Offspring phenotype is determined by the mother's genotype.

What is genomic imprinting?

Expression depends on whether an allele was inherited from the mother or father.

How can environment affect phenotype?

Temperature, nutrition, toxins, and other factors alter gene expression.

What are continuous traits?

Polygenic and environmentally influenced traits showing a range of variation.

Why is pedigree analysis important?

It helps determine inheritance patterns and predict genotypes.

What do twin studies show?

They estimate how much variation is genetic vs environmental.

What is linkage?

Genes close together on a chromosome tend to be inherited together.

What produces recombinant phenotypes?

Crossing over between homologous chromosomes.

How do you calculate recombination frequency?

(Recombinants ÷ Total offspring) × 100.

Qualitative vs quantitative traits?

Qualitative are categorical; quantitative show continuous variation.

What makes a trait quantitative?

Many genes with small additive effects plus environmental influence.

What are additive traits?

Traits where allele effects add up to produce the phenotype.

What is heritability?

The proportion of phenotypic variation due to genetic differences.

Broad-sense vs narrow-sense heritability?

Broad includes all genetic variance; narrow includes only additive variance.

What is allele frequency?

The percentage of each allele in the population.

What is the Hardy-Weinberg equation?

p² + 2pq + q² = 1.

What is inbreeding?

Mating between relatives that increases homozygosity.

What is genetic drift?

Random change in allele frequencies strongest in small populations.

What is the bottleneck effect?

A large population is suddenly reduced, decreasing variation.

What is the founder effect?

A small subgroup starts a new population with reduced variation.

What is fitness?

The reproductive success of a genotype compared to others.

How does natural selection differ from drift?

Selection is nonrandom; drift is random.

The paper discusses mosaic loss of the X chromosome (mLOX) in adult females. This phenomenon helps illustrate which broader genetics concept?

Dosage compensation through X-chromosome inactivation

Because mLOX arises when some cells lose an X chromosome during life it is an example of what genetics principle?

Somatic mosaicism

Some of the genetic variants the researchers identified were associated with the centromere or in proteins that interact with the centromere. How might those variants be connected to mLOX?

The variants associated with the centromere could cause improper segregation during anaphase stage of mitosis. This will result in cell loss which is how the X chromosome is lost in mLOX.

In Figure 5 the researchers describe their work to predict which of the two X chromosomes will be lost though mLOX. In your opinion how convincing are their results?

Yes, these results are convincing. They used data from over 56,000 cases from different biobanks and tested their predictions with over 27,000 mLOX cases. Additionally they had statistically significant predictions of associated health risks; these large numbers and significant results convinced us.

A woman is heterozygous for the sex-linked trait of haemophilia. Intriguingly she has a few sites on her body that bleed profusely when she is injured but other than that she shows no signs or symptoms of haemophilia. Explain.

X-inactivation silences normal copy in some cells

Which of the following does NOT contribute to phenotypic variance (Vp) within a population?

Number of offspring produced by an individual

How many Barr bodies would you expect to find in a XXY male and XXX female?

1 and 2, respectively

Manx cats have a greatly shortened (often absent) tail. This is a dominant trait. A cross of two manx cats produces 10 manx kittens and 5 normal tailed kittens. A second cross produces 6 manx kittens and 3 normal tailed kittens. Which of the following can you conclude about the manx trait?

It is homozygous lethal

Monozygotic twins form from the same zygote. Some twins are not identical - for example: some monozygotic twins differ in the severity of a particular disorder associated with mitochondrial electron transport. Which of the following can not be true with regards to this disorder?

The sperm was heteroplasmic for mitochondria

Which of the following statements is TRUE with regards to heritability (H or h)?

It is an indication of the genetic variability that contributes to a trait

For the 3 genotypes AA, Aa, aa: the number of progeny that survive to reproduce (on average) are 5 10 and 20 respectively. Which of the following statements is true?

The aa genotype has the greatest fitness