The chromosomal basis of inheritance Chapter 15

You must know How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance. The unique pattern of inheritance in sex-linked genes. How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders. How genomic imprinting and inheritance of mitochondrial DNA are exceptions to standard Mendelian inheritance.

Chromosome Theory of Inheritance Genes have a specific location (loci) on chromosomes Chromosomes segregate and assort independently

I. Sex – linked genes Genes located on the sex chromosomes (X or Y in humans) Discovered by Thomas Hunt Morgan while working with fruit flies

Inheritance of X-linked genes Males receive one copy of the X-linked gene from their mother and a Y chromosome (without that gene) from their father (can’t use homozygous/heterozygous) Females receive two copies of the gene – one from their mother and one from their father Traits represented as superscripts on the X

Most sex-linked traits/disorders are recessive Heterozygous females are called “carriers” – they don’t display the trait, but their sons may Females need two copies of the allele to express the trait, males only need 1 copy

Hemophilia – lack clotting factors in blood X-linked Disorders Duchenne muscular dystrophy – progressive weakening of the muscles, death by early 20s Hemophilia – lack clotting factors in blood Color blindness Interesting documentary on hemophilia and historical royal families if you’re interested (on your own time) https://www.youtube.com/watch?v=bmQwMllhCUM

X – inactivation Only one X chromosome worth of X-linked genes is necessary (as demonstrated by males only needing one) In females, one X (randomly selected) in each cell is inactivated by methylation Inactive X condenses into a Barr body

Linked genes Genes located in the same chromosome and are therefore more likely to be inherited together Only way they are not inherited together is if crossing over occurs between the two loci The farther apart two genes are on a chromosome, the more likely it is that crossing over will occur between them

Genetic recombination – production of offspring with a different combination of genes than either parent

Linkage Map – show where genes are located on a chromosome based on the percentage of crossover events (1 map unit = 1% recombination frequency)

Sample Problem In fruit flies, gray body is dominant to black, and normal wings are dominant to vestigial. Flies heterozygous for both traits were crossed with flies that were black with vestigial wings. The results are below. How do you know these genes are linked? How many map units apart are these two genes? Phenotype Number of flies Gray/normal 482 Black/vestigial 472 Gray/vestigial 103 Black/normal 92 Total = 1149 Recombinants = 195 195/1149 = 17% 17 map units

Alteration of chromosome number Nondisjunction – failure of homologous chromosomes or sister chromatids to separate properly during meiosis Trisomy = 3 copies of one chromosome Monosomy = 1 copy of one chromosome Results in aneuploidy – incorrect number of chromosomes

Human disorders from chromosomal alterations Most miscarried early in pregnancy (bigger the chromosome, more likely that is) Down Syndrome – Trisomy 21- characteristic facial features, short, heart defects, developmental delays

Klinefelter Syndrome – XXY – generally sterile, some developmental delays Turner Syndrome – XO – reproductive organs do not mature, sterile, developmental delays

Polyploidy – complete extra set(s) of chromosomes (3n, 4n, +) Rare in animals, but commons in plants Results in extra large fruits

Structural alterations of chromosomes Occur when crossing over is not completed correctly 1. Deletion – loss of a fragment of a chromosome 2. Duplication – extra copies of fragments of a chromosome

Inversion – part of a chromosome breaks off and reattached backwards 4. Translocation – a piece of one chromosomes breaks off and attaches to a non-homologous chromosome

Exceptions to standard Mendelian Inheritance Epigenetics – environmental factors influence gene expression

Exceptions to standard Mendelian Inheritance Genomic imprinting – traits vary depending on whether they are inherited from mom or dad During gamete formation, one allele is turned off by methylation

Exceptions to standard Mendelian Inheritance Mitochondrial DNA – only passed from mother to child