Midterm

Friday - or - Monday?

Covers Chapters 1,2 and 3.6

If yed, it will mean moving Exam 2 to May 6.

Sex Chromosomes

... ‘X’ and ‘Y’ chromosomes that determine the sex of an individual in many organisms,

Females: XX

Males: XY

Differential

Region

Differential Region

Paring RegionParing Region

male XX: female

a aA

male

a

hemizygous: condition where gene is present in only one dose.

X Linkage

…the pattern of inheritance resulting from genes located on the X chromosome.

X-Linked Genes

…refers specifically to genes on the X-chromosome, with no homologs on the Y chromosome.

Blue Female

Pink Male

xP

Gametesor

Gametesor

F1

Blue Female Blue Male

F1

Blue Female Blue Male

x

Gametesoror

Gametesoror

F2

Blue Female Blue Male Blue Female Pink Male

F2

Blue Female Blue Male Blue Female Pink Male

3 : 1

Pink Female

Blue Male

xP

Gametesor

F1

Blue Female Pink Male

Gametesor

F2

Pink Female Pink Male Blue Female Blue Male

Gametesoror

F2

Pink Female Pink Male Blue Female Blue Male

1 1 1 1

Sex Linkage to Ponder

• Female is homozygous recessive X-linked gene,

– what percentage of male offspring will express?

– what percentage of female offspring will express if,

• mate is hemizygous for the recessive allele?

• mate is hemizygous for the dominant allele?

• Repeat at home with female heterozygous X-linked gene!

Sex-Linked vs. Autosomal

• autosomal chromosome: non-sex linked chromosome,

• autosomal gene: a gene on an autosomal chromosome,

• autosomes segregate identically in reciprocal crosses.

X-Linked Recessive TraitsCharacteristics

• Many more males than females show the phenotype,– female must have both parents carrying the allele,

– male only needs a mother with the allele,

• Few (or none) of the offspring of affected males show the disorder,– all of his daughters are carriers,

• half of the sons born to these daughters are carriers.

X-Linked Dominant

• Affected males married to unaffected females pass the phenotype to their daughters, but not to their sons,

• Heterozygous females married to unaffected males pass the phenotype on to half their sons and daughters,

• Homozygous dominant females pass the phenotype on to all their sons and daughters,

Autosomal Dominant

• Phenotypes appear in every generation,

• Affected males and females pass the phenotype to equal proportions of their sons and daughters.

Recessive? ---> Yes!

Pedigree for Very Rare Trait

Autosomal?X-Linked? ---> Yes!

1/2

1/2

1/2 x 1/2 x ? 1/2 = 1/8

Linkage

• Genes located on the same chromosome segregate together and are said to be linked.

A

b2n = 4

Independent Assortment

a

A A

B

B b a B a b

Meiosis Prophase Icont.

Chiasmata and crossing over

A aA a

B B b b

a A

MeiosisNo Cross Over

Parent Cell

Daughter Cells Have Parental Chromosomes

A a

B b

A

B

a

b

a

b

A

B

2n = 1

MeiosisWith Cross Over

Parent Cell

Daughter Cells Have Recombinant Chromosomes

A a

B b

A

B

A

b

a

b

a

B

2n = 1

Gamate Formation in F1 Dihybrids P: GGWW x ggww

G g W w

GW Gw gW gw

alleles

gametes

probability.25 .25 .25 .25

F1 Genotype: GgWw

How do you test for assortment of alleles?

GW Gw gW gw

.25 .25 .25 .25

F1: GgWw

Test Cross

• phenotypes of the offspring indicate the genotype of the gametes produced by the parent in question.

Test CrossGgWw x ggww

GW (.25)

Gw (.25)

gW (.25)

gw (.25)

gw (1)

gw (1)

gw (1)

gw (1)

G gww (.25)

GgWw (.25)

ggWw (.25)

ggww (.25)

Test CrossGgWw x ggww

GW (.25)

Gw (.25)

gW (.25)

gw (.25)

gw (1)

gw (1)

gw (1)

gw (1)

Ggww (.25)

GgWw (.25)

ggWw (.25)

ggww (.25)

P

P

parental types GgWw and gwgw P: GGWW and ggww

R

R

recombinant types Ggww and ggWw

Recombination Frequency

…or Linkage Ratio: the percentage of recombinant types,

– if 50%, then the genes are not linked,

– if less than 50%, then linkage is observed.

Linkage

• Genes located on the same chromosome do not recombine,

– unless crossing over occurs,

• The recombination frequency gives an estimate of the distance between the genes.

Recombination Frequencies

• Genes that are adjacent have a recombination frequency

near 0%,

• Genes that are very far apart on a chromosome have a linkage ratio of 50%,

• The relative distance between linked genes influences the amount of recombination observed.

Dihybrid Cross

yellow/round green/wrinkled

GGWW x ggww

GW gw

GgWw

phenotype

genotype

gametes

genotype

P

F1

Questions

• Don’t rely on the answers in the back of the book to solve your problems…

– Don’t just solve them, but understand the principles needed to solve them.

• More practice questions are online. No answers are available, i.e. confirm your own answers

a

b

c

de

f