THEME 5 MOD 2

Question 1

what cells aren’t derived from mitosis

Answer 1

gametes - from specialized germ cells in ovaries / testes

Question 2

sexual reproducing organisms

Answer 2

• genes passed on to offspring through gametes
• offspring have genetic variation and inherit a unique combination of genes from both parents

Question 2

production of gametes

Answer 2

• replication of parental sex cell precursors
• two rounds of meiotic cell division to make diploid number of chromosomes haploid

Question 3

human male production of gamete

Answer 3

4 haploid sperm cells from one precursor sex cell

Question 3

human female production of gametes

Answer 3

one sex cell precursor will produce one large egg cell and 3 non-gamete polar bodies which store extra chromosomal content

Question 4

why arent human gametes produced by mitosis

Answer 4

• gametes would be diploid and give rise to offspring with double the chromosomes, leading to fatal effects in the offspring

Question 5

How are gametes unique and variable within a parent

Answer 5

• recombination of parental homologous chromosomes

Question 5

meiosis

Answer 5

• duplication of chromosomes
• 2 consecutive cell divisions resulting in 4 haploid daughter cells

Question 6

Steps of meiosis I

Answer 6

interphase: duplication of chromosomes

meiosis I:
prophase I:
- condensation and synapsis (pairing and physical connection) of homologous chromosomes along their length
- all homologous chromosomes pair with each other and form a bivalent unit (pair of synapsed chromosomes, four stranded structure)
- synaptonemal complex facilitates connection between two homologous chromosomes
- cross over or recombination between homologous chromosomes (specifically between non-sister chromatids), evident at the chiasma or chiasmata(plural) between paired chromosomes
- cross over of homologous chromosomes results in breaking and rejoining between non-sister chromatids
- recombinant chromatids result in a combination of paternal and maternal segments of dna
- centromere duplication, movement, spindle formation and nuclear envelope breakdown occur

metaphase I: spindle apparatus randomly reorders pairs of homologous chromosome bivalents relative to each other, further adding to genetic diversity between gametes

anaphase I: proteins holding homologous pairs together break down (synaptonemal complex) and homologues of bivalents pulled to opposite polls

telophase I marks end of meiosis I: each half of the cell has a haploid set of duplicated chromosomes: each chromosome has a recombinant pair of sister chromatids

chromosomes slightly uncoil, nuclear envelope reforms

after meiosis I cell begins cytokinesis into two separate cells

Question 7

sister chromatids vs homologous chromosomes

Answer 7

sister chromatids: pair from duplicated chromosome
non-sister chromatids: two pairs of sister chromatids, one of each parental origin, non sister chromatids have similar genetic material, but aren’t identical
Homologous chromosomes: individual chromosomes that have been inherited from each parent with different alleles of genes on the loci of each chromosome

Question 8

steps of meiosis II

Answer 8

Meiosis II: equational division (each daughter cell has equal number of chromosomes
prophase II: 23 different and duplicated sister chromatids
- nuclear envelope breaks down, spindle apparatus forms, chromosomes condense

metaphase II: chromosomes positioned at the metaphase plate
anaphase II: proteins at centromere break down, allows sister chromatids to sperate to opposite pollsas individual chromosomes
Telophase II: end of meiosis II. separate duplicated sister chromatids. nuclear envelope reforms, chromosomes condense

cytokinesis follows

Question 9

nondisjunction

Answer 9

IN MEIOSIS
failure of chromosomes to separate during meiosis I or sister chromatids to separate a meiosis II

• can result in gamete with one less or one extra chromosome, can lead to detrimental or lethal effects

IN MITOSIS: same principle, cause cancerous cells

Question 10

mitosis vs meiosis

Answer 10

meiosis: very similar
differences:
- one interphase
- anaphase I: sister chromatids dont seperate, homologous chromosomes do
- meiosis II only has 23 chromosomes

Question 11

gregor mendel

Answer 11

• studied pea plants and inheritance of traits, declaring two laws of inheritance
• discovered basis of inheritance before the structures of chromosomes were discovered
• studied 7 traits: seed colour, seed shape, seed pod shape and colour, flower colour, flower position, plant height

Question 12

basis of mendel experiments

Answer 12

• started with two true breeding plants (over generations, offspring were always the same variety of parent). homozygous plants carried two identical allele
• crossed true breeding plants then cross bred offspring to determine any mathematical patterns to inheritance

Question 13

how was an allele deemed recessive or dominant

Answer 13

when two true-breeding/homozygous plants were crossed, which phenotype showed
- cross between true-breeding yellow and green plants resulted in all yellow F1 generation, meaning yellow allele was dominant
- disproved blended model of inheritance

Question 14

P generation, F1, F2

Answer 14

P: parental
F1: filial (son) 1
F2: filial 2

Question 15

P: yellow and green true breeding plants
F1: all yellow
F2: …

Answer 15

dominant to recessive seed colour (and all traits) : 3:1

3/4 F2 phenotype: dominant for all traits

1/4 F2 phenotype:
recessive for all traits

Therefor, heritable factor of green plant masked in F1

Question 16

mendels first law

Answer 16

• law of segregation
• alleles of a gene segregate into different gamete formation
• based upon: alternate forms of genes accountting for variation in inherited traits
• offspring will inherit two versions of the gene (one from both parents) and the dominant one will appear as a phenotype
• if organism is true breeding, allele found in all four gametes

Question 17

punnet square

Answer 17

• consider all possible gametes of each parent
• predict offspring genotype and phenotype

Question 18

how gametes get one specific allele

Answer 18

• homologous chromosomes equally segregated into gametes, each containing one allele of a gene
• if an organism has identical alleles at that gene locus across both chromosomes, each gamete will have the same allele

Question 19

heterozygous plant probabilities in offspring

Answer 19

50% chance passing off Big A or little a

F2: 50% chance of getting big A allele from one parent and 50% chance of getting big A from other. Probability of inheriting AA: 50% times 50%= 25% (same for aa)

F2: probability of heterozygous genotype is twice that of the homozygous genotype as it can occur in two combinations. heterozygous genotype probability: 50%

underlying genotypic ratio of 1:2:1

Question 20

how to determine underlying genotype?

Answer 20

• cross individual with recessive individual to determine if individual is hetero or homozygous
• if heterozygous: when crossed with homozygous recessive: half of the offspring will be heterozygous for the yellow allele, and half will be green (recessive homozygous)
• if homozygous: all offspring will be heterozygous for the yellow allele
• heterozygous genotype occurred 2:1 with homozygous genotype when crossing individuals, supporting 1:2:1 genotype ratio

Question 21

monohybrids and monohybrid crosses definition

Answer 21

MONOHYBRIDS:
cross breed of two true breeding plants
F1: all heterozygous for the one trait followed in the cross
MONOHYBRID CROSSES:
cross between two heterozygous plants in F1

Question 22

second law of inheritance

Answer 22

law of independent assortment - explains cross ratios observed by Mendel - dependant on how nonhomologous chromosomes align the metaphase plate: equally probabilities of different orientations, meaning hypothetically 1:1:1:1 ratio of all possible gametes at end of meiosis II - only the case when genes are located on different chromosomes - if genes are close enough together on a chromosome they will be inherited together and called linked genes P: cross bred plants that had yellow and wrinkled seeds with plants that had green and smooth seeds F1: all plants had yellow and smooth seeds, as these were the dominant alleles F2: 9 yellow-round: 3 green round: 3 yellow wrinkled: 1 green wrinkled dihybrid phenotype ratio

Question 23

Explain the 9:3:3:1 dihybrid phenotype ratio

Answer 23

- 50% chance of gamete containing a or A, 50% chance of gamete containing b or B - 4 possible gamete combinations aabb: - only one way this can be the case, if both parents have ab allele - 1/4 times 1/4 = 1/16 AaBb: - probability of one parent having ab is 1/4, probaility of another parent having AB: 1/4 (1/4 times 1/4= 1/16) - however, four distinct ways to get AaBb - add up probabilities: 1/16 +1/16+1/16+1/16= 1/4

Question 24

explain B gene phenotype

Answer 24

B gene, later identified as SBE1 gene - dominant allele produces smooth seed - recessive allele produces wrinkled seeds - SBE1 gene codes for starch branching enzyme (converts sugars into starch) and is interupted in wrinkled seeds - bb allele seeds have double the sugar of BB seeds - therefor wrinkled seeds have a higher sugar and water content and are larger, however, as the water is lost during maturation, the excess sugar causes a wrinkled phenotype