Patterns of Inheritance: X-Linked Inheritance and Independent Assortment

Flashcards covering X-linked inheritance, chromosomal sex determination systems, Drosophila crosses, pedigree analysis, and independent assortment of alleles at two different genes.

X-Linked Inheritance

The pattern of inheritance for genes that are physically located on the X chromosome.

Sex Chromosomes

The chromosomes (such as the larger X chromosome and smaller Y chromosome in mammals) that determine biological sex and function as a homologous pair during meiosis.

Autosomes

All chromosomes other than sex chromosomes that are present in two copies in both biological males and females (e.g., $22$ pairs in humans).

X0 System

A chromosomal sex-determination system found in some insects where males have a single X chromosome and no second sex chromosome.

ZW System

A chromosomal sex-determination system found in birds where the female has two different sex chromosomes (Z and W) and the male has two copies of the Z chromosome.

Haploid-Diploid System

A sex-determination mechanism operating in eusocial insects where sex is governed by chromosome ploidy level rather than specific types of sex chromosomes.

Hemizygous

A term describing an individual having only a single copy of a gene or chromosome, such as human males having only one copy of any gene on the X chromosome.

Wild Type Allele ($w^+$)

The normal, non-mutant allele of a gene, such as the red-eye allele in Thomas Hunt Morgan's Drosophila crosses.

X-Linked Recessive Inheritance

A mode of inheritance where a recessive allele on the X chromosome requires two copies to be expressed in females, but only a single copy in hemizygous males to show the phenotype.

X-Linked Dominant Inheritance

A mode of inheritance where a single copy of a dominant disease-causing allele on the X chromosome produces the phenotype; affected males transmit the trait to all daughters and no sons.

Carrier

An unaffected individual who does not exhibit the disease phenotype but possesses one copy of a recessive disease-causing allele (a heterozygote).

Hemophilia

An X-linked recessive blood disorder traced through Queen Victoria's royal family pedigree, transmitted through unaffected carrier females (such as Alice and Alix) to affected males like Alexis.

Independent Assortment

The principle that alleles of two different genes on different chromosomes segregate independently into gametes during meiosis due to the equally frequent alternative alignments of chromosome pairs.

Double Heterozygote

An individual that is heterozygous for two different genes, which produces four different gamete genotypes in equal proportions during meiosis.

$9:3:3:1$ Phenotypic Ratio

The phenotypic ratio observed in the $F_2$ generation from crossing two double heterozygotes for two genes located on different chromosomes with simple dominant and recessive alleles.

$1:1:1:1$ Phenotypic Ratio

The phenotypic ratio obtained when crossing a double heterozygote with a double homozygous recessive individual for two independently assorting genes.

Linked Genes

Genes located on the same chromosome that tend to be inherited together through meiosis and do not show independent assortment unless separated by large chromosomal distances.