Concepts in genetics - exam 2

Bisc 303

Punnett square

visualizes the segregation and random union of alleles

What does independent assortment in crosses of F1 dihybrids produces

a 9:3:3:1 phenotype ratio

Each F1 dihybrid produces what

four possible gametes in a 1:1:1:1 ratio

Four phenotypic classes occurred in the F2 progeny:

• Two are like parents

• Two are recombinant

crosses of pure-breeding lines can result in what

progeny phenotypes that don’t appear to follow Mendel’s rules

Major mechanisms for these variations:

• No definitively dominant or recessive allele

• More than two alleles exist

• Multiple genes involved (gene interactions, epistasis)

• Gene-environment interactions

The process of solving extensions of Mendel problems

Diagram the cross in a consistent manner

Three questions to cover when solving extensions of Mendel problems

1. How many genes are involved in determining the phenotype?

2. How many alleles of each gene are present?

3. What phenotypes are associated with which genotypic classes?

Extensions to Mendel for single-gene inheritance

1. Dominance is not always complete

2. A gene may have (many) more than two alleles

3. Pleiotropy

Incomplete dominance

Phenotypes and Phenotypic ratio

Codominance

Phenotypes and Phenotypic ratio

Pleiotropy

one gene may contribute to several characteristics

A gene can have more than two alleles

1. Multiple alleles of a gene can segregate in populations

2. Each individual can carry only two alleles

3. Dominance relations are always relative to a second allele and are unique to a pair of alleles

Dominance relations between alleles do not affect transmission of alleles

still sex-based reproduction meiosis and fertilization by gametes

Multiple allele example

Seed coat patterns in lentils are determined by a gene with five alleles

gene interaction

two genes can interact to determine one trait

what is a result of gene interactions

Novel phenotypes

Complementary gene action

fewer phenotypes maybe observed due to

Epistasis

when an allele at one gene masks the phenotype fo alleles at another gene

Genes that perform the same function are

redundant

Epistasis

effects of a gene mask the effects of another

Bombay phentoype

another example of recessive epistasis

Dominant Epistasis in a dihybrid cross ratio

12:3:1

Dominant inhibitory epistasis (dominant suppression)

13:1 phenotypic ratio

Redundant Gene function

15:1 ratio

Heterogenerous traits

many genes give rise to a phenotype

Heterogenous traits have

the same phenotype but are caused by mutations in different genes

Complementation testing

used to determine if a particular phenotype arises from mutations in the same or separate genes

When can Complementation testing be applied

only with recessive, not dominant, phenotypes

Multifactorial traits

• Genes can interact to yield novel phenotypes

• Gene interactions can display epistasis, where an allele of a gene can mask the effects of another gene

• One trait can be influenced by many different genes

Penetrance

the percentage of a population with a particular genotype that show the expected phenotype

Expressivity

degree with which a genotype is expressed in a phenotype

Phenotypes can show

variation in both penetrance and expressivity

Environmental effects on phenotypes:

Temperature affects survivability of a Drosophila mutant

Conditional lethal mutations are lethal only under some conditions:

• Permissive conditions

• Restrictive conditions

Permissive conditions

mutant allele has wild-type functions

Restrictive conditions

mutant allele has defective functions

Each genes that contributes to a continuous or quantitative trait are referred to as

quantitative trail loci or QTLs

Gamete contain what

½ the number of chromosomes as the zygote

Haploid cells

carry only a single chromosome set

Diploid cells

carry two matching chromosome sets

n

the number of chromosomes in a haploid cell

2n

the number of chromosomes in a diploid cell

Autosomes

pairs of nonsex chromosomes

How are sex chromosomes and autosomes arranged

in homologous pairs

A human has how many sex chromosomes and autosomes

22 pairs of autosomes and 1 pair of sex chromosomes

Before meiosis, testes cells had 24 chromosomes

22 in matched pairs (autosomes) and 2 unmatched (large = X and smaller = Y)

After meiosis two types of sperm were formed:

• ½ of sperm had 11 chromosomes and an X

• ½ of sperm had 11 chromosomes and a Y

After meiosis, only one type of egg was produced

all had 11 chromosomes plus an X

Sex chromosome

• provide basis for sex determination in humans

• One sex has matching pair

• Other sex has one of each type of chromosome

Sex determination in humans

children receive only an X chromosome from mother but X or Y from father

What is the role of the SRY gene

determines maleness in humans

Sex determination in fruit flies

the ratio of X chromosomes to autosomes determines gender

Sex determination in humans

presence or absence of Y chromosome determines gender

Abnormal numbers of X or Y chromosomes have

different effects in humans and flies

Heterogametic sex

gender with two different kinds of gametes (XY males in humans, ZW females in birds)

Homogametic sex

gender with one type of gamete (XX females in humans, ZZ males in birds)

Meiosis

chromosomes replicate once, nuclei divide twice

Oogenesis

egg formation in humans

Diploid germ cells

called oogonia, multiply by mitosis to produce primary oocytes

Primary oocytes

undergo meiosis I to produce one secondary oocyte and one small polar body (which arrests development)

Secondary oocyte

undergoes meiosis II to produce one ovum and one small polar body

polar bodies

discard excess genetic material while conserving almost all of the cytoplasm, nutrients, and cellular machinery for the single surviving egg cell (ovum)

What does Symmetrical meiotic division produces

four functional sperm

Spermatogenesis in humans

begins in male testis in germ cells called spermatogonia

Mitosis produces what during spermatogenesis

diploid primary spermatocyte

Meiosis I produces what during spermatogenesis

two secondary spermatocytes per cell

Meiosis II produces what during spermatogenesis

four equivalent spermatids

Spermatids mature into

functional sperm

Wild-type allele

allele that is found in high frequency in a population (denoted with a “+’)

Mutant allele

allele found in low frequency (denoted with no symbol)

Recessive mutation

gene symbol is in lower case

Dominant mutation

gene symbol is in upper case

Rare events of nondisjuntion in XX female produce

XX and O eggs

X recessive traits in humans are identified by

• 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 heterozygous

• Trait often skips a generation

• Trait only appears in successive generations if sister of an affected male is a carrier.

X dominant traits in humans are identified by

• trait appears in more females than males

• Trait is seen in every generation

• All daughters, but non of the sons from an affected male will be affected. (Most distinguishing characteristic of dominant X-linked trait)

• Sons and daughters of an affected female each have a 50% change of being affected

X inactivation causes

females to be mosaic

Sex-limited traits

genes that affect a structure or process not found in the other sex

Sex influenced traits

expression of a trait that differs between sexes

Recombinant progeny can arise when

two genes on non-homologous chromosomes assort independently during gamete formation or by recombination between two genes on homologous chromosomes.

Linkage and meiotic recombination

• Genes liked together on the same chromosome usually assort together

• Linked genes may become separated through recombination

Mapping

the frequency with which linked genes become separated reflects the physical distance between them

Independent assortment

genes on different chromosomes

Linkage

two genes on same chromosome segregate together

Crossing over between homologous chromosomes leads to

separation of linked genes

Genes on the same chromosome assort together more often than not, In dihybrid cross:

departures from a 1:1:1:1 ratio of F1 gametes indicate that the two genes are on the same chromosome

In a sex linked cross F2 males receive

their X chromosome from their mothers. Hence their phenotype directly indicates the gametes produced in the heterozygous F1 female.

Detecting linkage by analyzing the progeny of dihybrid crosses

X-linked genes

Syntenic genes

genes located on the same chromosome

Linkage in an autosomal gene

• genotypes of F1 female gametes are revealed by test cross

• Parental class outnumbers recombinant class demonstrating linkage

Reciprocal exchanges between homologous chromosomes are the

physical basis of recombination

Recombination helps what

chromosome segregation

Recombination frequencies for pairs of genes reflect what

the distance between them

Unliked genes show a recombination frequency of

50%

When genes are linked which one is less frequent

recombinant gametes are less frequent than parental gametes

Recombination frequencies between two genes

never exceed 50%

The RF of unlinked genes is

50% due to independent assortment

The RF of linked genes cannot exceed 50%

• meioses without crossovers produce only parental chromosomes

• Single and double crossover produce a 1:1 parental to recombinant chromosome ratio on average

Genes close together on the same chromosome

are linked and do not segregate independently

Linked genes lead to what

a larger number of parental class than expected in double heterozygotes