3.7.1 Inheritance

One refers to the actual genes, while the other refers to what is physically expressed or observed.

Answer: The genotype is the genetic constitution of an organism, whereas the phenotype is the expression of this genetic constitution and its interaction with the environment.

The genotype is the genetic constitution of an organism. The phenotype is the expression of this genetic constitution and its interaction with the environment.

Think of human blood types, where someone can have both A and B antigens (Type AB).

Answer: Both alleles are equally expressed in the phenotype of a heterozygous individual.

Alleles may be dominant, recessive or codominant. Codominant alleles both contribute to the phenotype without one fully masking the other in a heterozygous state.

Females are $XX$ and males are $XY$. The $Y$ chromosome is much smaller and lacks many gene loci found on the $X$.

Answer: Males only have one $X$ chromosome, so a single recessive allele on this chromosome will be expressed in the phenotype.

In sex-linkage, males ($XY$) possess only one allele for $X$-linked genes. Therefore, a recessive allele cannot be masked by a dominant allele on a second $X$ chromosome, as it can in females ($XX$).

An autosome is any chromosome that is not a sex chromosome. If genes are on the same physical piece of DNA, they travel together.

Answer: Two or more genes are located on the same autosome (non-sex chromosome) and tend to be inherited together.

Autosomal linkage refers to genes being located on the same autosome. Because they are on the same chromosome, they do not assort independently during meiosis unless crossing over separates them.

An example is a gene for albinism overriding a gene for hair colour; if the organism is albino, the hair colour gene's alleles don't matter.

Answer: An interaction between genes where the allele of one gene masks or alters the expression of alleles of a different gene.

Crosses involving... epistasis. Epistasis occurs when the expression of one gene depends on or is masked by a different, independently inherited gene.

You perform this test to see if the offspring numbers from your genetic cross match the Mendelian predictions.

Answer: The goodness of fit of observed phenotypic ratios with expected theoretical ratios.

Use of the chi-squared ($X^2$) test to compare the goodness of fit of observed phenotypic ratios with expected ratios. It tests if the difference between observed and expected data is due to chance.

This involves two genes sorting independently, producing 16 possible combinations in the Punnett square.

Answer: $9:3:3:1$

A standard dihybrid cross involving two heterozygous individuals (e.g., $AaBb \times AaBb$) yields a $9:3:3:1$ expected phenotypic ratio.

Human blood groups (ABO) are a classic example.

Answer: There are more than two different alleles of that specific gene in the population, although an individual can still only possess two.

There may be many alleles of a single gene (multiple alleles) in a population. However, any diploid individual will only inherit two of those alleles (one on each homologous chromosome).