Lecture 7: Meiosis and the Chromosomal Theory of Inheritance

Question 1

what did Walter Sutton discover?

Answer 1

studied great lubber grasshopper:
- body cells contained 22 chromosomes and X & Y chromosomes
- gametes contained 11 chromosomes and X or Y in equal numbers
- after fertilisation, cells with XX were females and cells with XY were males

Question 2

anatomy of a chromosome

Answer 2

• metaphase chromosomes are classified by the position of the centromere: metacentric (if it is in the middle of the chromosome) or acrocentric (if located close to an end)
• sister chromatids are held together by centromeres
• homologous chromosomes are a pair of chromosomes—one from each parent—that have the same size, shape, and genes at the same locations, but may carry different versions (alleles) of those genes.

Question 3

draw metacentric and acrocentric chromosomes

Answer 3

slide 5

Question 4

key stages of mitosis

Answer 4

Replication:
Interphase - cell grows and DNA is copied
Prophase – Chromosomes condense, spindle forms

Segregation:
Metaphase – Chromosomes line up in the middle
Anaphase – Chromatids are pulled apart
Telophase – New nuclei form

Cytokinesis – Cell splits in two genetically identical 2n daughter cells

Question 5

key stages of meiosis

Answer 5

Interphase – DNA is copied
Prophase I – Homologous chromosomes pair up and cross over
Metaphase I – Pairs line up in the middle
Anaphase I – Homologous chromosomes separate
Telophase I – Two nuclei form

Prophase II – New spindles form in each cell
Metaphase II – Chromosomes line up in the middle
Anaphase II – Chromatids are pulled apart
Telophase II – Four nuclei form
Cytokinesis – Four haploid cells result

Question 6

key differences between mitosis and meiosis

Answer 6

Number of divisions:
Mitosis = 1
Meiosis = 2

Number of daughter cells:
Mitosis = 2 (diploid)
Meiosis = 4 (haploid)

Genetic similarity:
Mitosis = Identical to parent
Meiosis = Genetically different (variation)

Purpose:
Mitosis = Growth, repair, asexual reproduction
Meiosis = Sexual reproduction (gametes)

Homologous chromosomes:
Mitosis = Do not pair
Meiosis = Pair and crossover in Prophase I

Question 7

how was movement of chromosomes during mitosis and meiosis understood ?

Answer 7

microscopy provided a means to follow movement of chromosomes during cell division

Question 8

gamete

Answer 8

contains one-half the number of chromosomes as the zygote

Question 9

haploid

Answer 9

cells that carry only a single chromosome set

Question 10

diploid

Answer 10

cells that carry two matching chromosome sets

Question 11

n

Answer 11

the number of chromosomes in a haploid cell

Question 12

2n

Answer 12

the number of chromosomes in a diploid cell

Question 13

is chromosome number constant across species?

Answer 13

no, it varies from species to species but does not correlate with the size or complexity of the animal

Question 14

somatic cells

Answer 14

divide mitotically and make up the vast majority of an organism’s tissues

Question 15

germ cells

Answer 15

specialised role in the production of gametes:

Question 16

process of creation of a diploid offspring

Answer 16

• germ cells arise during embryonic development in animals and floral development in plants
• undergo meiosis to produce haploid gametes
• gametes unite with gamete from opposite sex to produce diploid offspring

Question 17

karyotype

Answer 17

• produced by cutting micrograph images of stained chromosomes and arranging them in matched pairs
• sex chromosomes and autosomes are arranged in homologous pairs

Question 18

autosomes

Answer 18

pairs of non-sex chromosomes

Question 19

sex chromosome

Answer 19

• provide basis for sex determination
• one sex has matching pair, other sex has non-matching sex chromosomes
• variation in sex determination between species

Question 20

mammal and drosophila sex chromosomes

Answer 20

female: XX
male: XY

Question 21

some grasshoppers sex chromosomes

Answer 21

female: XX
male: XO

Question 22

fish, birds, moths sex chromosomes

Answer 22

female: ZW
male: ZZ

Question 23

sex determination in humans

Answer 23

children receive only one X chromosome from mother but either X or Y from father

Question 24

X chromosomes in females

Answer 24

females typically have two X chromosomes that are genetically identical

Question 25

how are chromosome behaviour and inheritance related?

Answer 25

chromosomes determine characteristics of organism (eg its sex); therefore, basis of inheritance must reside there

Question 26

mitosis ensures that

Answer 26

every cell in an organism carries the same set of chromosomes

Question 27

meiosis ensures that

Answer 27

one member of each chromosome pair is distributed to gamete cells

Question 28

gametogenesis

Answer 28

the process by which germ cells differentiate into gametes

Question 29

6 links between chromosome and gene behaviour

Answer 29

- each cell contains 2 copies of each chromosome; each cell contains 2 copies of each gene - chromosome complements appear unchanged during transmission from parent to offspring; genes appear unchanged during transmission from parent to offspring - homologous chromosomes pair and then separate to different gametes; alternative alleles segregate to different gametes - maternal and paternal copies of chromosome pairs separate without regard to the assortment of other homologous chromosome pairs; alternative alleles of unrelated genes assort independently - at fertilisation an egg's set of chromosomes unite with randomly encountered sperm's chromosomes; alleles obtained from one parent unite at random with those of another parent ; alleles obtained from one parent unite at random with those of another parent - in all cells derived from a fertilised egg, one half of chromosomes are of maternal origin, and half are paternal; in all cells derived from a fertilised gamete, one half of genes are of maternal origin, and half are paternal

Question 30

Thomas hunt morganatic

Answer 30

- started using a new genetic model: drosophila melanogaster, the common fruitfully - in 1910, morgan discovered a white-eyed male among his true-breeding stocks of red-eyed flies (first drosophila mutation identified)

Question 31

advantages of drosophila as a model organism

Answer 31

- small size - short generation time of 10 days at room temp - each female lays 400-500 eggs - easy to culture in laboratory - small genome - large chromosomes - many mutations available

Question 32

draw a table describing how sex determination in humans differs from drosophila

Question 33

nomenclature for drosophila genetics

Answer 33

wild type allele: allele that is found in high frequency in a population (greater than or equal to 1%) is denoted with a + mutant allele: allele that is found in low frequency (less than 1%): denoted with no symbol recessive mutation: gene symbol is in lower case dominant mutation: gene symbol is in upper case

Question 34

examples of notations for drosophila

Answer 34

- Cy, Sb, D are dominant mutations - vg, v, y, e are recessive mutations - vg+ - wild type allele for vestigial gene locus - Cy+ - wild type allele for curly gene locus

Question 35

eye colour in drosophila

Answer 35

- flies with red eyes carry the wild type allele, w+ - any variation in eye colour carry a mutant allele (eg white eye colour), w

Question 36

how was X-linkage of eye colour in drosophila demonstrated?

Answer 36

1. observed true-breeding sex ratios (1/2 male, 1/2 female in both F1 and F2) 2. P cross: crossed a white-eyed male (w/Y) to a red-eyed female (w+/w+). all F1 offspring had red eyes, suggesting the white-eyed trait was recessive 3. F1 cross: crossed w+/w female to w+/Y male. all female offspring were red-eyed; male were white and red-eyed. thus, morganatic proposed that the eye colour gene is on the X chromosome.

Question 37

reciprocal crosses done to confirm X linkage

Answer 37

reciprocal cross of w/w females with w+/Y males. F1 was red-eyed females and white-eyed males only. F2 cross of w+/w females with w/Y males resulted in half offspring having white eyes, half having red eyes. females 1:1 red: white, males 1:1 red:white.

Question 38

sex linkage

Answer 38

genes for specific traits are carried on sex chromosomes

Question 39

hemizygous

Answer 39

means having only one copy of a gene instead of the usual two (eg in males with X-linked genes)

Question 40

what was Bridges' hypothesis and how did it arise?

Answer 40

X chromosomes were not segregating properly in meiosis, leading to 1/2000 females being white-eyed and 1/2000 males being red-eyed during a w/w x w+/Y cross

Question 41

how was Bridges' hypothesis proved?

Answer 41

he did large enough experiments to observe rare meiotic events he called non-disjunction.

Question 42

draw 3 diagrams: - normal X chromosome segregation - nondisjunction in meiosis I - nondisjunction in meiosis II

Answer 42

slide 34

Question 43

how did nondisjunction lead to unexpected phenotypes in drosophila?

Answer 43

males receive one X chromosome from father: red eyes females receive two X chromosomes from mother: white eyes

Question 44

what was the final evidence that phenotypes are associated with chromosomes and thus that genes are carried on chromosomes?

Answer 44

- Bridges crossed XXY females (white eyes) to the normal XY males (red eyes) in order to confirm his theory - unusual inheritance patterns correlate with aneuploidy

Question 45

aneuploidy

Answer 45

abnormal number of chromosomes

Question 46

X-linked recessive traits exhibit 6 characteristics seen in pedigrees

Answer 46

- Trait appears in more males than females. - Mutation and trait never pass from father to son. - Affected male does pass X-linked mutation to all daughters, who are then unaffected carriers. - Trait often skips a generation. - Trait only appears in successive generations if sister of an affected male is a carrier. If so, her sons have a 50% chance of showing the trait. - All sons of affected female show trait and all daughters of affected female are carriers.

Question 47

example of X-linked recessive trait in humans

Answer 47

haemophilia A

Question 48

X-linked dominant traits exhibit 4 characteristics seen in pedigrees

Answer 48

- Trait appears in more females than males. - Sons and daughters of an affected heterozygous female have a 50% chance of showing the trait. - Trait is seen every generation. - All daughters but no sons of affected male show trait.

Question 49

example of X-linked dominant trait in humans

Answer 49

hypophosphatemia

Question 50

Y-linked traits exhibit 3 characteristics seen in pedigrees

Answer 50

- Trait appears only in males. - All sons but no daughters of affected male show trait. - Females do not show and cannot transmit trait.