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

Chromosome theory of inheritance: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

Thomas Hunt Morgan Studied fruit flies – Drosophilia melanogaster

Fruit Flies are excellent for genetic studies because: Reproduce quickly Easy to raise Many mutations Have 8 chromosomes (n=4)

Fruit Fly Mutations                                

Thomas Hunt Morgan began to carry out experiments with

Morgan looked at TWO traits Gray bodies – G Normal Wings - W Black bodies – g Small wings – w

The flies mated….

The female laid eggs                                                   

GGWW ggww P1 x F1 GgWw 100%

Morgan then mated the F1 back to the recessive parent GgWw x ggww Expected ratio – 1:1:1:1 25% GgWw 25% Ggww 25% ggWw 25% ggww

Morgan’s Actual Results 41.5% gray normal 41.5% black small 8.5 % black normal 8.5% gray small

Conclusion Gene for body size and wing color were somehow connected or linked Can’t undergo independent assortment

Linkage Groups Package of genes that are always inherited together Chromosome One linkage group for each homologous pair Fruit flies – 4 linkage groups Humans – 23 linkage groups Corn – 10 linkage groups

So linkage groups explain the high percentages (41.5%) but What about the 8.5%??????

The combinations that were expected would be: 17% had new combinations The combinations that were expected would be: Gray normal – GW or Black small - gw

P1 G G g g W W w w Dad Mom

F1 G g W w

g G g g W w w w F1 F1 F1 X Recessive Fruit Fly Heterozygous

The Offspring of the Cross and W w w w F1 F1 41.5 % 41.5 %

Genes of the Heterozygous Parent W W w w The homologous pair copied

The homolgous pairs pair up in Prophase and form a tetrad W W w w

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

The other offspring of the cross and w w W w F1 F1 8.5 % 8.5 %

The 17% that had new combinations are known as Recombinants – individuals with new combinations of genes Crossing Over – gives rise to new combinations – Prophase I

Gene Mapping Sturtevant – associate of Morgan Crossing over occurs at random The distance between two genes determines how often they cross over Genes that are close do not crossover often Genes that are far apart – cross over often

So…… If you know the frequency with which crossing over occurs then you can use that to map the position of the genes on the chromosome

Frequency of crossover exchange...                            is GREATER the FARTHER apart 2 genes are    is proportional to relative distance                                       between 2 linked genes    Relative distance is established as...        1% crossover frequency =                                   1 map unit of map distance        1%   CrossOver  Freq   =    1   centiMorgan

Sex Chromosomes One pair Female – XX Male – XY                  

Sex Linkage Stevens – made observations of meal worm chromosomes

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 genes 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

Sex linked genes Male pattern baldness Hemophelia colorblindness

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 offspring

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 change 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 nonhomologous 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 nucloetide, 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

Codominace Occurs when both alleles for a gene are expressed in a heterozygous offspring Neither allele is dominant of 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

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