Genetics exam 1

Characteristic vs. trait

Characteristic: seed color

Trait: green/yellow

Monohybrid cross

P (parental) = pure breeding

F1: monohybrids, heterozygous for one gene

F2: 3:1

Law of segregation

Each individual possesses two alleles encoding a characteristic, which segregate during gamete formation in equal proportions

Testcross

cross to homozygous recessive individual

Independent assortment

Alleles for two genes segregate independently

Four phenotypic classes in F2 of a dihybrid cross:

9:3:3:1

Multihybrid individual

2^n gametes, n= number of genes for which individual is heterozygous

Phenotype probability

3/4= probability of dominant phenotype (AA or Aa)

¼ = probability of recessive phenotype

Number of possible scenarios for unordered series of events

n!/x!y!z! -

n= total # of individuals

x,y,z= number of individuals in each category

multiply by prob in non-ordered

Neither prob: do individual probabilities 1- and then multiply

Chi squared

DF= number of terms-1

sum of observed-expected squared over expected

Pedigrees with dominant traits

Trait cannot skip a generation

Two affected parents can produce unaffected children if both are heterozygotes

Pedigrees with recessive traits

Trait can skip a generation

Two affected parents cannot produce unaffected children

Pedigrees with rare recessive traits

Assume that fewest genetically unrelated individuals carry the same affected allele

Assume that an unaffecteed individual is not a carrier unless:

there is evidence they had an ancestor with the mutation

it is impossible to explain pedigree without them being carrier (they have affected child)

exam may assume only one uncertain carrier parent

P(carrier given unaffected)

Incomplete dominance

hybrid resembles neither parent but a mix between two

AA→ red

Aa→ pink

aa→ white

F2 phenotypic ratio of two pure breed cross in P generation is 1:2:1

Codominance

hybrid displays traits of both parents

CrCr→ red

CrCw→ red and white

CwCw→ white

1:2:1 ratio in F2

Multiple alleles for a gene

A gene can have more than two alleles, each individual can still only carry two alleles

Dominance series:

> means dominance

= means either codominance or incomplete dominance

A1>A2>A3=A4>A5

Dominance relations are established between pairs of alleles

Recessive lethal alleles

Awt

A*= lethal

AwtAwt-alive

AwtA*-alive

A*A*- dead

A* is dominant to Awt in determining a trait, but recessive for lethality

Pleiotropy

one gene determines several distinct and seemingly unrelated characteristics

ex: some Maori men have respiratory problems and are sterile

Recessive mutation in a gene that codes for protein required for cilia (failure to clear lungs) and flagella (immotile sperm)

Novel phenotypes resulting from gene interaction, seed coat in lentils

9:3:3:1 ratio in F2 suggests two independently assorting genes for a characteristic, two genes function in independent parallel pathways

Redundant genes

15:1 phenotype ratio

(9) A_B_ normal

(3) A_bb normal

(3) aaB_ normal

1 aabb- new trait

AAbb x aaBB F1 all identical- AaBb x AaBb

Complementary gene action

two individuals affected with a recessive trait have unaffected offspring, come from pure breeding lines

9 A_B_

3 A_bb

3 aaB_

1 aabb

9:7

Dominant/ functional allele in both genes

Epistasis

an allele at one gene masks the effects of another gene

Recessive epistasis- epistatic allele must be homozygous recessive

Dominant epistasis- one copy of an allele masks the other gene

Recessive: 9:3:4 ratio

9 B_E_

3 bbE_

4: 3 B_ee, 1 bbee

ee masks the effect of all B genotypes

Dominant:

12:3:1

12: 9 A_B_, 3aaB_

3: A_bb

1: aabb

Genotype B_ masks the effects of all A genotypes

aa and Aa determine trait when bb present

Dominant II

13:3

9 A_B_

3 aaB_

1 aabb

3 A_bb

Genotype B_ masks the affects of allele A

Dominant epistasis indicates antagonistic functions, one protein makes pigment and another prevents its deposition

Redundant genes

Duplicate dominant epistasis, two genes perform the same function, mutant phenotype when both knocked out

15:1

9 A_B_

3A_bb

3 aaB_

1 aabb

Complementary gene action

Duplicate recessive epistasis

Twp different genes both required to produce normal phenotype, recessive mutation in either gene disrupts pathway→ mutant

9:7

9 A_B_ normal

7: 3 A_bb, 3 aaB_, 1 aabb mutant

Penetrance

the perentage of individuals with a particular genotype that show the expected phenotype

Not everyone who gets allele gets phenotype

Expressivity

Phenotype develops to different extents

Environmental effects on phenotype: temperature

Siamese cat- warmer temperature = nonfunctional enzyme, no melanin

cooler temperature= functional, melanin, permissive conditions

Conditional lethal allele

Non-permissive condition- dead

Ex: Drophsolia mutant Shibire^ts

T<29 degree C, protein works, neurons work → viable

T>29, protein denatures→ inviable

ABO blood types in humans- rare Bombay phenotype

Peoploe with hh genotype will test as type O regardless of ABO genotype: two parents with blood type O can produce a child type A or B

Normal human karyotype

46 = 23 pairs, 22 pairs of autosomes + 1 pair of sex chromosomes

ploidy=number of sets of autosomes in a cell

Diploid cell (2n) n=3 ploidy = 2

Haploid gametes (n) n=3 ploidy = 1

Cell cycle

Cohesion: ring-shaped protein molecules holding sister chromatids together

Centromere: physical constriction, most cohesion is located there

Interphase: G1 growth, S Dna replication sister chromatids, G2 more growth and prep

Mitosis: Prophase: chromosomes condense and become visible

Metaphase: sister chromatids attach to microtubules from opposite poles, chromosomes align on metaphase plate

Anaphase: sister chromatids separate and move towards opposite poles

Telophase: daughter cells form

Cytokinesis: separation

Kinetochore: attachment site for microtubules that pull sister chromatids apart

Meiosis

Meiosis I

homologs separate, reductional division, ploidy is reduced from 2n to n

P1: homologs pair, cross over and for bivalents (pairs)

M1: bivalents align

A1: homologs segregate

Metaphase II

PII

MII- pairs of sisters align

AII: sisters segregate

sister chromatids segregate

equational division

Crossing over forms bivalent, held together by sister chromatid cohesion and chiasma (site of crossing over)

Cohesion during meiosis I and II

Cohesion binds chromatids to form bivalent, cohesion removed from arms by separase but is protected at centromeres by sugoshin

Anaphase I- homolog pairs separate and sisters go together, sugoshin degraded

Metaphase II- centromeric cohesion cleaved, sisters go apart in Anaphase II

Metaphase I: monopolin binds sister kinetochores and makes them behave as one, monopolar attachment of sisters

II: bipolar attachment of sisters

Meiotic nondisjunction creates abnormal gametes

MI: homologs go together and do not separate into daughter cells, 4 abnormal cells, 2 have n+1 and 2 have n-1

MII nondisjunction: sister chromatids do not separate, one daughter cell has extra chromosome, sister chromatids fail to separate in 1 of the two divisions, one gets n+1 and one gets n-1 chromosomes, others are normal

Aneuploidy- extra or missing chromosomes

Polyploidy: multiple complete sets of chromosomes

Meiosis and genetic diversity

Independent assortment of nonhomologous chromosomes creates different combos of alleles located on diff chromosomes Metaphase I

Crossing over between homologs in prophase I creates diff combos of alleles within each chromosome

Linked genes

Alleles travel together, located close together on one chromosome

Can be separated by recombination (crossing over)

Syntenic genes: located on same chromosome

Recombination frequency

#recombinbant gametes/#total gametes x 100%

Genes linked if number of gametes with parental genotypes exceeds the number with recombinant genotypes

Never exceeds 50% between two genes

Distance between genes

Mutant alleles can be in coupling or repulsion

cis/trans

DCO

two crossover events occur between same pair of homologs, if happen at same loci can cancel each other out and restore parental config, combo happened but did not detect it, @ longer distances become more frequent

Sex-linked trait

gene is located on sex chromosome

X linked is dominant or recessive

Y linked is hemizygous

X-linked recessive traits

Trait can skip generations, hides in female carriers

Trait cannot be passed from father to son

If female is affected, all her sons must be affected, all daughters carriers

X-linked dominant

Trait cannot skip generation

Trait cannot be passed from father to son

If male is affected, all his daughters must be affected and none of his sons can be affected

Y-linked

Trait cannot skip generations

Only males affected

All sons of an affected male must be affected

Females do not exhibit trait and cannot be carriers

Autosomal genes and sexual dimorphism

Sex-influenced traits: sex influences phenotype of the heterozygote

B1B2 male: beard

B1B2 female: no beard

Sex-limited traits: limited to one sex

sk- autosomal mutation in Drosophila, alters male anatomy

often dominant in one sex and recessive/less penetrant in the other

Human sex chromosomes

PAR- pseudoautosomal region for both sexes

SRY- sex determining region of Y, determines maleness, TF

MSY- male specific region of Y

XO turner XXX poly x- female

XXY klin XYY poly ymale

Random X inactivation

each cell randomly inactivates one X chromosomes in early mammalian female embryo

Ex: color in cats

Barr body- condensed inactive X chromosome in nucleus of cell from normal female

Each cell is hemizygous (one copy instead of two, leading to that one always being expressed) All but one X chromosomes are inactivated in each cell, almost all genes are silent

Heterozygous females ONLY- random x inactivation creates some cells expressing the normal allele some expressing mutant , if skewing favors normal no symptoms

Drosophila, # X chromosomes determines sex

X: male

XO, XYY

Female: greater than or equal to 2 Xs

XXY

XXX

XXXY

Mechanism of sex determination differ between species

C. elegans:

XX- hermaphrodites

XO-males

Birds/butterflies

ZW: females (heterogametic sex)

ZZ males (homogametic sex)

Lizards, alligators, turtles, tortoises- environmental sex determination (temperature)

Haplodiploid sex determination

Diploid 2 female, haploid n male

Crisscross inheritance: trait appears in one sex in one gen and the opposite in next gen with no father to son transmission directly

Sons get all genes from mother

Fathers contribute genes only to daughters

Tetrad analysis in yeast

Diploid yeast cell after meiosis produced 4 haploid spores