unit 3: genetics

Unit 3: Genetics

• The Cell Cycle + DNA structure/function• Mitosis and Meiosis• Mendelian Genetics (aka - fun with Punnett squares)

• DNA replication

• Create and complete two testcross Punnet squares:

(assume G=green and g=yellow)

gg x Gg gg x GG

½ green all green½ yellow

• Why homozygous recessive for testcross?–Clear and easy determination of unknown’s genotype: 1:1 = heterozygote; all dominant = homozygote

Yesterday’s Exit Ticket

G G

g

g

Gg Gg

Gg Gg

G g

Ggg

g Gg

gg

gg

Today’s Agenda:

• Mendel and multiple characters •Exceptions to Mendel• Sex-linked traits• Gene linkage

https://www.youtube.com/watch?v=Y1PCwxUDTl8

How To Punnet Squares

Fig. 14-8

RESULTS

F1 Generation

Predictions

Hypothesis ofdependentassortment

YyRr

Hypothesis ofindependentassortment

orPredictedoffspring ofF2 generation

Sperm SpermYRYR yryr Yr

YRyR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

Phenotypic ratio 9:3:3:1

1/21/2

1/2

1/2

1/4

yr

1/41/4

1/41/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

2. Probability and genetic outcomes2. Probability and genetic outcomes

What about multiple characters? Are they inherited together or separately?

For the purposes of example, consider the following two characters:

1. Seed color:• Possible phenotypes = Yellow OR green• Yellow is dominant to green

2. Seed shape:• Possible phenotypes = Round OR wrinkled• Round is dominant to wrinkled

Fig. 14-8

EXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes


YYRR yyrr

YR yr

YyRr


orPredictedoffspring ofF2 generation

Sperm SpermYRYR yryr Yr

YRyR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr


EggsEggs


1/21/2

1/2

1/2

1/4

yr

1/41/4

1/41/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16


What about multiple characters? Are they inherited together or separately?

Important Vocab Note:A MONOHYBRID cross deals with one gene

e.g. Aa x Aa

A DIHYBRID cross deals with two genese.g. AaBb x AaBb

Fig. 14-8

EXPERIMENTP Generation

F1 Generation

Gametes

YYRR yyrr

YR yr

Suppose that two F1 individuals are crossed. Consider two mutually exclusive hypotheses about inheritance:

1. Strict dependent assortment = inherited allele combinations are ALWAYS preserved in the gametes an individual produces

2. Independent assortment = all possible combinations of inherited alleles of different genes are equally likely in an individual’s gametes

Fig. 14-8

EXPERIMENT

RESULTS

P Generation

F1 Generation

Gametes


YYRR yyrr

YR yr

YyRr


or

Predictedoffspring ofF2 generation

Sperm

SpermYR

YR

yr

yr

Yr

YR

yR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr


EggsEggs


1/21/2

1/2

1/2

1/4

yr

1/41/4

1/41/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

Phenotypic ratio approximately 9:3:3:1315 108 101 32


Fig. 15-2b

0.5 mm

Meiosis

Metaphase I

Anaphase I

Metaphase II

Gametes

1 4yr 1 4

Yr14 YR

3 3

F1 Generation: 2 possiblearrangements ofchromosomes

1 4yR

R

R

R

R

RR

R

R R R R

R

Y

Y

Y Y

Y

YY

Y

YY

YY

yr r

rr

r r

rr

r r r r

y

y

y

y

y

y y

yyyy

All F1 plants produceyellow-round seeds (YyRr)

1

2 2

1

LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.

LAW OF SEGREGATIONThe two alleles for each gene separate during gamete formation.

LAW OF SEGREGATIONThe two alleles for each gene separate during gamete formation.

• Mendel’s “law” of independent assortment = alleles for each character segregate independently during gamete formation

• Given what YOU know about the relationship between genes and chromosomes (which Mendel did NOT), when would this “law” be violated?



Y

yr

R

RY

ry

Today’s Agenda:

• Mendel and multiple characters • Exceptions to Mendel• Sex-linked traits• Gene linkage

If only it were all so simple…If only it were all so simple…

The view provided by (my simplified presentation of) Mendel’s pea experiments:

• one gene one character(e.g., flower color gene color of flower)

• one allele one phenotype(e.g., P allele purple flower)

• two alleles of each gene, one completely dominant, the other recessive(e.g., P dominant to p)

• Patterns of inheritance different from those discussed so far can be caused in many ways. Just to name a few:a) Lack of complete dominance by one alleleb) A gene has more than two allelesc) A gene produces multiple phenotypesd) Multiple genes affect a single phenotypee) Environmental circumstances affect the phenotype

3. Extending the Mendelian model3. Extending the Mendelian model

To learn more about all of these, take EBIO 2070!

For now, the simplest exceptions:

1. Genes on sex chromosomes2. Gene linkage

Y

X

REMINDER:

A complete single set of human chromosomes

includes:

• 22 autosomes (non-sex chromosomes)

• 1 sex chromosome

(diploid cells have 44 autosomes and

2 sex chromosomes)

Fig. 15-5

X

Y

Humans and many other species have chromosomal sex determination

In the human system,females have two “X” chromosomes, males have one “X” and one “Y”

Fig. 15-6c

The Z-W system

76 + ZW

76 + ZZ

Other forms of chromosomal sex determination in the animal kingdom…

What consequences might sex chromosomes have for patterns of inheritance and gene expression?

22 pairs of chromosomes

+

X Y

22 pairs of chromosomes

+

X X

Who determines the sex of our offspring?

XXDiploid

Parent Cell

Gametes X X

XY

X

Dad determines a child’s sex!

Y

from female parent from male parent

allele on X chromosome(“X-linked”)

passed on to either sons or daughters with

probability ½

passed on ONLY to daughters with

probability 1

Patterns of inheritance in mammals (and other XY systems)

XAXaDiploid

Parent Cell

Gametes XA Xa

XAY

XA Y

from female parent from male parent

allele on Y chromosome(“Y-linked”)

typically not possessed by females

passed on ONLY to sons with probability 1

Patterns of inheritance in mammals (and other XY systems)

XXDiploid

Parent Cell

Gametes X X

XYA

X YA

Dad is who determines a child’s sex

expression in females expression in males

dominant X-linked allele yes yes

recessive X-linked allele

ONLY if present with other recessive allele yes

Y-linked allele never present(never expressed) yes

Patterns of gene expression in mammals (and other XY systems)

Male-pattern baldnessSRY gene:

Testes formation

Santhi’s Story

http://www.ibnlive.com/videos/28851/how-are-athletes-gender-tested.html

Santhi Soundarajan won the silver medal in the 800-meter race at the 2006 Asian Games in Doha, Qatar.

Following her silver medal performance, she was stripped of her medal.

Santhi has female genitalia but her genotype is XY.

Speakequal.com

expression in females expression in males

dominant X-linked allele yes yes

recessive X-linked allele

ONLY if present with other recessive allele yes

Y-linked allele never present(never expressed) yes

Patterns of gene expression in mammals (and other XY systems)

• Breeding fruit flies (Drosophila melanogaster)

– Rapid breeders

– Males = XY; Females = XX

– For Drosphila:recessive alleles = “mutant” (b)dominant alleles = “wild type” (b+)

Genes on chromosomesGenes on chromosomesb) Sex-linked traitsb) Sex-linked traits

News.wisc.edu

Fig. 15-3

One of Morgan’s experiments (think back to Mendel’s peas):• Character: eye color• Phenotypes: red or white

Is the allele for white eyes dominant or recessive?

P Generation(true breeding)

F1 Generation All offspring had red eyes

Then, cross the F1 offspring with each other, and what does the F2 generation look like?

3:1 ratio of red : white

2:1:1 ratio of red female : red male : white male

3:1 ratio of red : white

2:1:1 ratio of red female : red male : white male

The best explanation for the pattern of inheritance seen in the F2 generation is:

a)The eye color gene is on an autosomeb)The eye color gene is sex-linked, on the X chromosomec)The eye color gene is sex-linked, on the Y chromosomed)There is not enough information to discriminate between hypotheses (a) through (c)

Fig. 15-4c

EggsF1

CONCLUSION

Generation

Generation

P XX

r

Sperm

XY

EggsSperm

GenerationF2

R

r

r

RR

r

R

R

RR

r

R

R

rr

Genes on chrom

osomes

Genes on chrom

osomes

b) Original discoveries


r

All females XRXr All males XRY

Fig. 15-4c

EggsF1

CONCLUSION

Generation

Generation

P XX

r

Sperm

XY

EggsSperm

GenerationF2

R

r

r

RR

r

R

R

RR

r

R

R

rr

Genes on chrom

osomes

Genes on chrom

osomes



r

All females XRXr All males XRY

Females all red: ½ XRXr

½ XRXR

Males half red (XRY) and half white (XrY)

For now, the simplest exceptions:

1. Genes on sex chromosomes2. Gene linkage

Y

X

https://www.youtube.com/watch?v=-_UcDhzjOio

Gene Linkage and Fruit Flies

Fig. 15-2b

Meiosis

Metaphase I

Anaphase I

Metaphase II

1 4Yr

3

1 4yR

R

R

R

R R

R

Y

Y

YY

Y

Y

yr

r

r

r

r r

y

y

y

yy

All F1 plants produceyellow-round seeds (YyRr)

2

1

LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.

Fig. 14-8

F1 Generation


YyRr


or

Predictedoffspring ofF2 generation

Sperm

SpermYR

YR

yr

yr

Yr

YR

yR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr


EggsEggs


1/21/2

1/2

1/2

1/4

yr

1/41/4

1/41/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

In crosses involving two characters, sometimes you get outcomes that were intermediate between

these two hypotheses.

• Morgan crossed flies to study the characters of body color and wing size Genes for both are located on autosomes

Example

Fig. 15-9-1

EXPERIMENT P Generation (homozygous)

Wild type(gray body,normal wings)

Double mutant(black body,vestigial wings)

b b vg vg b+ b+ vg+ vg+

F1 generation

?

Fig. 14-8

F1 dihybrid(wild type phenotype)

b+ b vg+ vg



xF1 dihybrid

(wild type phenotype)

b+ b vg+ vg

3 : 1 9 : 3 : 3 : 1

Observed (approx.): 8 : 2 : 2 : 4

b+b?v+v? bbvv b+b?v+v? b+b?vv bbv+v? bbvv

• Each chromosome has hundreds or thousands of genes• Genes located on the same chromosome that tend to be inherited

together are called linked genes

• Occasional crossing over leads to occasional, but not common, recombinant chromosomes

Why would some genes be inherited neither completely together nor completely independently?

Gene linkage

Prophase Iof meiosis

Pair ofhomologs

Nonsisterchromatidsheld togetherduring synapsis

Chiasma

Centromere

Anaphase I

crossing over Sources of genetic variation

Recombination of Linked Genes: Crossing Over

Crossing over during Prophase I of meiosis is the mechanism for recombining alleles Crossing over during Prophase I of meiosis is the mechanism for recombining alleles

Fig. 15-UN1

b+ vg+

Parents in testcross

Most offspring

b+ vg+

b vg

b vg

b vg

b vg

b vg

b vg

or

Gene linkage

Fig. 15-10b

Testcrossoffspring

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

b+ vg+

b vg b vg

b vg b+ vg

b vg

b vg

b+ vg+

Spermb vg

Parental-type offspring Recombinant offspring

b vg

b+ vg b vg+

b vg+

Eggs

Recombinantchromosomes

3. Gene linkage

5 : 5 : 1 : 1

An x-linked recessive allele b produces red-green color blindness in humans. A normal-sighted woman whose father was color-blind marries a color-blind man.

1.What genotypes are possible for the mother of the colorblind man?

1.What are the chances that the first child from this marriage will be a color-blind boy?

1.Of the girls produced by these parents, what proportion can be expected to be color-blind?

1.Of all the children (sex unspecified) of these parents, what proportion can be expected to have normal color vision?

Today’s Exit Ticket

unit 3: genetics

Documents

eggseggsphenotypic ratio

yellow gg x gg gg x

multiple characters

possible phenotypes

genetic outcomeswhat

inherited allele combinations

aabb x aabbfig

f1 individuals