genetics lect 5 2011 colour 2 slides per page

8/13/2019 Genetics Lect 5 2011 Colour 2 Slides Per Page

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Lecture5

LECTURE 5

Genetics: the chromosomal basis of

inheritance I

BIOSCI101Essential

1. Mendelian inheritance has its physical basis in the

behaviour of chromosomes

2. Sex-linked traits indicated that genes are on

chromosomes

3. Linked genes

4. Genetic recombinationiology

(i) recombination of unlinked genes; independent

assortment of chromosomes(ii) recombination of linked genes; crossing over

5. Mapping a chromosome s genetic loci using

recombination

Lecture5

GENETICS CYTOLOGY

1. Mendelian inheritance has i ts physical

basis in the behaviour of chromosomes

BIOSCI101Essential

1860

1870

1880

1890

Cytologists work out mitosis

(1875) & meiosis (1890s)

Mendel proposes that discrete

inherited factors segregate and

assort independently during

gamete formation (1857)

iology 1900

1910

Mendels principles are

rediscover

Sutton, Boveri and other saw the parallels between Mendelian

genetics and chromosomes in meiosis 1902.


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Lecture5

Chromosomes and genes are both present in pairs in

diploid cells.

Marriage of genetics and cytology

BIOSCI101Essential

Homologous chromosomes separate and allele pairs

separate during meiosis.

Fertilization restores the paired state of chromosomes

and genes.

Concluded - chromosome theory of inheritanceiology

1. Mendelian factors have specific positions on

chromosomes2. it is the chromosomes that segregate &

independently assort.

Lecture5

Mendels first law: the principle of

segregation

The two alleles segregate during gamete

production e.g. the sperm and egg eachBIOSCI101Essential

carry one allele.

Gene pair

Rr

iology

Diploid state is restored by fusion of

gametes

R r


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Lecture5

Mendels second law: the principle of

independent assortment

Each allele pair assorts

BIOSCI101Essential

during gamete formation.

The alleles of unlinked genes assort

independently.

iology

Rr

Gene pair

Yy

Lecture5

The chromosome basis of Mendels laws

BIOSCI101Essential

iology


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Lecture5

2. Sex-linked traits indicated that genes

are on chromosomes

BIOSCI101Essential

iology

Thomas Hunt Morgan used the fruit fly to locate the

1st gene to a chromosome

Lecture5

Morgans first mutant

BIOSCI101Essential

iology

Fruit flies have three pairs of autosomes and one pair

of sex chromosomes (X & Y)


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Lecture5

Morgans first mutant (continued)

BIOSCI101Essential

iology

F2 showed a typical Mendelian 3:1 ratio, however all

white-eyed flies were males.

Lecture5

Specific genes are on specific chromosomes

BIOSCI101Essential

iology


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Lecture5 The number of genes per genome far exceeds the

3. Linked genes

BIOSCI101Essential

number of chromosomes, therefore there must be more

than one gene per chromosome.

Linked genes are genes located on the same

chromosomes and that tend to be inherited together.

iology

Linked genes are exceptions to Mendelian inheritance.

Lecture5

Are the genes for body colour and wing

size linked?

BIOSCI101Essential

Parents in

testcross

iology

Fruit flies: vg = vestigial wings, b = black, + = wild type & - = mutant.

Offspring if

linked


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Lecture5

BIOSCI101Essential

iology

If the two genes are on different chromosomes then we

should see equal number of the four types of offspring.

Lecture5

Recombination of linked genes

b vg

vBIOSCI101Essential

b vg

b vg

b+ vg+

b vg

bvgb+vg+

b vg+

b+ vg

b vg

iology

b+ vg+ b+ vg+

b+ vg+

b vg

b vg


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Lecture5

4. Genetic recombination

Recombination of unlinked genes:

occurs by independent assortment.

BIOSCI101Essential

on different chromosomes or when genes are far

apart on the same chromosome.

Recombination of linked genes:

iology

u y v u .

linked genes do not assort independently i.e.observe less than 50% recombinants; this is

because genes are close together on the same

chromosome.

Lecture5

5. Mapping a chromosomes loci by using

recombinant frequencies

Assumed that if crossing over occurred randomly then

the frequency that crossing over occurs should beBIOSCI101Essential

proportional to the distance between two genes.

Used frequency of recombinants to build linear

chromosome maps.

iology

chromosome.

1 map unit = 1% recombinant frequency


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Lecture5

BIOSCI101Essential

iology

Lecture5

Calculating map distances

Offs rin Number BIOSCI101Essential

Recombinants206 + 185

= 391

Total965 + 944 + 206 +

185 = 2300

iology Map distance

(391 / 2300) x 100= 17 %


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Lecture5

The observed recombination frequencies between threefruit fly gene pairs are b-cn 9%, cn-vg 9.5%, & b-vg 17%.

Constructing a linkage map

BIOSCI101Essential

iology

The b-vg recombination frequency is slightly less thanthe sum of the b-cn and cn-vg because double crossing

overs can occur reducing the recombination frequency.

genetics lect 5 2011 colour 2 slides per page

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