The Chromosomal Basis of Inheritance

Chapter 15

Review

• Mitosis• Meiosis• Chromosome• Genotype and Phenotype• Mendelian Genetics

Thomas Hunt Morgan

• Studied Drosophila melanogaster– Large number of

offspring– Small, easy to care for– 4 chromosomes with

easily observable phenotypes

Drosophila melanogaster Phenotypes

• Wild type: red eyes• Variations: eye color, body color, wing shape• Males and females are easy to tell apart

Morgan’s Cross

• Red eyed female x White eyed male (w+) (w)

• F1 100% red eyed (wild type)

• F2 3:1 ratio BUT only males had white eyes

Morgan’s Conclusion

• The gene for eye color must be carried on the X chromosome and NOT an autosome

• Higher probability of a male having the recessive phenotype

Sex Linked Traits

Color Blindness

• Xb = No color blindness• XB = color blindness

• Determine the crosses (both phenotype and genotype) for the following crosses

1. Color blind father x normal mother yields one color blind son.

2. Normal father, carrier mother.3. Normal father, color blind mother.4. What cross will yield a color blind daughter?

Sex Linked Traits

• Called hemizygous • Do occur in females but males have a higher

probability of inheriting the trait• Duchenne muscular dystrophy, hemophilia,

color blindness

X Inactivation in Females

• Having two X chromosomes is a lot of genes!• One X chromosome will be turned off – Barr body: X chromosome condenses and will be

near the nuclear envelope– Ovaries – Barr body will be duplicated for viable egg

cells– In development, different X chromosomes could be

turned off • About ½ of the cells display the mother’s traits and half

display the father’s

Review of Pedigrees and Blood Typing

Linked Genes

• Genes on the same chromosome that tend to be inherited together

• Morgan believed body color and wing shape were inherited together

Genetic Recombination

• Recombinant: Offspring show combinations of traits not found in the parents

• How does this happen?

Recombinants

• Mendel’s peas• YyRr x yyrr– Complete the punnett square for the above cross.

• Which genotype and phenotypes are recombinants? Which are parental types?

Crossing Over• During Meiosis I (Prophase I)• Homologous chromosome pairs come

together forming a tetrad– Crossing over• Each chromosome will cross with the other in the pair• Parts of the chromosome will be exchanged

Recombination Frequency

• How likely is it that the two genes will be linked?

• Based on how close they are on the chromosome– Closer they are, more likely they will be linked

• Linkage map: genetic map based on recombination frequency

Linkage Mapping Problem

Mating: AaBb x aabb • A = Long antennae• a = Short antennae• B = Green eyebrows, and• b = Blue eyebrows.

Say you make this mating, and your actual results look like this: • Long Green - 850 • Long Blue - 150 • Short Green - 150 • Short Blue - 850

Calculate Linkage Map Distance

• One linkage map unit (LMU) is 1% recombination. Thus, the linkage map distance between two genes is the percentage recombination between those genes.

• In this case, we have a total of 300 recombinant offspring, out of 2000 total offspring. Map distance is calculated as (# Recombinants)/(Total offspring) X 100.

• What is the LMU of the two genes?

Linkage Map Summary

1. How can you recognize when genes are linked?

2. How do you calculate linkage map distance? 3. What does the linkage map distance tell you?4. How does a linkage map relate to

independent assortment of genes?

Linkage Map for Drosophila melanogaster

Dihybrid Cross

• In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d). If a squash plant true-breeding for white, disk-shaped fruit is crossed with a plant true-breeding for yellow, sphere-shaped fruit, what will the phenotypic and genotypic ratios be for the F1 generation and the F2 generation?

F1

F2

Summary of Dihybrid Cross

• White color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d)

Chi Square (X2) Test

• Determines statistical significance of a set of data

• Use in genetics: if genes sort INDEPENDENTLY they will follow the expected ratios of domintant : recessive– If not, they will not follow expected ratios and we

would reject the null hypothesis

Chi Square Set UpPhenotype Genotype #observed #expected observed –

expected (o-e)

(o-e)2 (o-e)2 / e

Monohybrid Cross Practice Problem• A student makes a monohybrid cross with Drosophila.

She crosses two heterozygotes for the white eye. Ww x Ww. She expects to see a 3:1 phenotypic ratio of Red eyes (WW and Ww) to white eyes (ww), her null hypothesis. She rears the next generation through to adult flies and counts the following numbers: – White eyes 210 – Wild type 680

• What change occurs in the allelic frequencies between generations 1 and 2? Perform a chi square analysis on these results and find out if it is close enough to 3:1 to fail to reject her null hypothesis. Make sure to show all work and explain your conclusions.

Dihybrid Chi Square ProblemIn the garden pea, yellow cotyledon color is dominant to green, and inflated pod shape is dominant to the constricted form. Considering both of these traits jointly in self-fertilized dihybrids, the progeny appeared in the following numbers:– 193 green, inflated– 184 yellow constricted– 556 yellow, inflated– 61 green, constricted

Do these genes assort independently? Support your answer using Chi-square analysis.

Baby chicks are pecking at grains of cracked corn of different colors. A researcher is attempting to determine if they have a preference for one color over another. He counts the number of grains chosen in an arena that has equal numbers of red, blue, green and yellow corn chunks. Here are his results:

– red: 250 – blue: 220 – green 230 – yellow 240

Perform a Chi Square analysis on these results. Can he reject the null hypothesis of no preference? Explain your answer.

Exit Ticket

15.4 Alterations of chromosome number or structure cause some

genetic disorders

Abnormal Chromosome Number

• Nondisjunction: chromosomes do not separate correctly– Meiosis I or II

Aneuploidy

• Gamete with abnormal number of chromosomes unites with a normal gamete

• Offspring will have abnormal number of chromosomes

• Monosomic (2n – 1)• Trisomic (2n + 1)• Polyploidy: triploidy (3n), tetraploidy (4n)

Chromosomal Mutations

• Involve changes in the number or structure of chromosomes

• 4 types:– Deletion– Duplication– Inversion– Translocation

Deletion and Duplication

• Deletion:– Loss of all or

part of a chromosome

• Duplication:– Produce extra

copies of the chromosome

Inversion and Translocation

• Inversion:– Reverse direction of

parts of the chromosome

• Translocation:– Part of one chromosome

breaks off and attaches to another chromosome

Human Disorders caused by Chromosomal Alterations

• Turner Syndrome• Down syndrome• Cri du chat• Prader-Willi Syndrome• Huntington’s Disease• Angleman Syndrome

Inheritance Patterns

• Not all fall into set patterns of equal inheritance

• Genomic Imprinting– Effect of the allele for a certain trait depends on

which parent passed on the trait– Could be expressed in different strengths

Organelle Genes

• Extranuclear genes (found in cytoplasm)• Mitochondrial genes • Chloroplast genes• NOT distributed to the offspring in the

Mendelian fashion• Come from mother – why?

Mitochondrial Genes

• Most make up the ETC and ATP synthase• Defects in these genes will affect energy

production– Most severely nervous system and muscular

system