Chapter 10

10.1 The Chromosome Theory of Heredity Chromosomes are located in the nucleus Factors (genes) are found on chromosomes Sutton discovered that genes are on chromosomes in 1902

Chromosome Theory of Heredity States that genes are located on chromosomes and each gene occupies a specific place on a chromosome Only one allele is on a chromosome

Independent Assortment

Gene Linkage Genes on a chromosome are linked together Inherited together – THEREFORE they do not undergo independent assortment

Linked Genes- genes on the same chromosome – inherited as a package Height Gene A Flower color gene B Flower position gene C

Linkage Groups Package of genes that are always inherited together Chromosome One linkage group for each homologous pair

When they are lined up they can become twisted and switch genes Crossing Over

So you could then have ….. G G g g W w W w switch

Recombinants – individuals with new combinations of genes Crossing Over – gives rise to new combinations – Prophase I

Sex Linkage Stevens – made observations of meal worm chromosomes

Sex Chromosomes One pair Female – XX Male – XY                  

Autosomes All the chromosomes except the sex chromosomes

Sex Determination

Genes on Sex Chromosomes Sex chromosomes determine a person’s sex Sex chromosomes also contain genes

Sex Linked A gene located on a sex chromosome Usually X Example – Fruit Fly Eye Color So the gene for eye color is on the X chromosome and not the Y

Fruit Fly Sex Chromosomes

Females Males XRY XrY XRXR XRXr XrXr Red Eyed White Eyed

Mutations

A change in the DNA of an organism Can involve an entire chromosome or a single DNA nucleotide and they may take place in any cell

Germ Cell Mutation Occur in an organism’s germ cells (gametes)- can only affect offpsring

Somatic Mutations Take place in an organisms body cells and can affect the organism

Lethal Mutation Cause death, often before birth

Good Mutations Some mutations can be beneficial – these organisms have a better chance to reproduce and therefore have an evolutionary advantage Provide the variation on which natural selection acts

Chromosome Mutations

Are either changes in the structure of a chromosome or the loss of an entire chromosome or an addition Four Types (duplication, deletion, inversion and translocation)

Duplication – segment of a chromosome is repeated Deletion – the loss of a chromosome or part due to chromosomal breakage – that information is lost

Inversion – a chromosomal segment breaks off and then reattached in reverse orientation to the same chromosome

Translocation – a chromosome breaks off and reattaches to another non-homologous chromosome

Nondisjunction Some chromosome mutations alter the number of chromosomes found in a cell Nondisjunction – the failure of a chromosome to separate from its homologue during meiosis

Gene Mutations

May involve large segments of DNA or a single nucleotide within a codon Involve individual genes

Point Mutations – 3 types The substitution, addition or removal of a single nucleotide

Substitution – a point mutation where one nucleotide in a codon is replaced with a different nucleotide, resulting in a new codon Ex. Sickle Cell Anemia – sub. Of A for T in a single codon

2 & 3. Insertion and Deletions – one or more nucleotides is lost or added – have more serious effects

Frameshift Mutation When a nucleotide is lost or added so that the remaining codons are grouped incorrectly Insertions and deletions are frameshift mutations

THE FAT CAT ATE THE RAT

Polyploidy Condition in which an organism has an extra set of chromosomes 3N, 4N Usually fatal in animals Plants – usually more robust Caused by - Nondisjunction

10-3 Regulation of Gene Expression As biologists have intensified their studies of gene activity, it has become clear that interactions between different genes and between genes and their environment are critically important

Gene Interactions Gene – piece of DNA – DNA codes for proteins In many cases the dominant allele codes for a protein that works and the recessive allele codes for a protein that does not work

Incomplete Dominance When offspring have a phenotype that is in-between the two parents Occurs when two or more alleles influence the phenotype Example – flowers – four o’ clocks, snapdragons Alleles – R/R’, R/r, R/W, FR F r

Red Flower

White Flower

Pink Flower Red mixed with white makes pink

Incomplete Dominance Example #2 Incomplete dominance is a half way between point. Halfway to dark blue is light blue.

Incomplete Dominance is not a blending.

RR rr Rr

Phenotypic Ratio: 1:2:1 Genotypic Ratio: 1:2:1

A Cross Example Cross two pink four o clocks. Label the phenotypes.

More problems Cross a red four o’clock with a pink four o’clock. Cross a red four o’clock with a white four o clock. Cross a red eyed male fruit fly white a white eyed female fruit fly.

Codominace Occurs when both alleles for a gene are expressed in a heterozygous offspring Neither allele is dominant or recessive Example – horse coat color

Horse Coat Color Red – HR HR White – HWHW Roan – HR HW

Roan – red and white hairs

Blue roan - The coat has white hairs and blue hairs

A Cross Example Cross two roan horses. Label the phenotypes.

Some more problems Cross a roan horse with a white horse. Cross a red horse with a white horse.

Polygenic Inheritance Traits controlled by two or more genes Examples – height, skin color, coat patterns Phenotypes are seen in a range

Polygenic Inheritance AB Ab aB ab AABB AABb AaBB AaBb AAbb Aabb aaBB aaBb aabb