genetics lect 6 2011 colour 2 slides per page

8/13/2019 Genetics Lect 6 2011 Colour 2 Slides Per Page

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Lecture6

LECTURE 6

Genetics: the chromosomal basis of

inheritance II

BIOSCI101Essential

Key concepts

1. The chromosomal basis of sex

2. X chromosome inactivation

3. Non-disjunction and alterations of chromosome

numberiology

. era ons o c romosome s ruc ure

5. Human disorders due to alterations of chromosome

number

Lecture6 1. The chromosomal basis of sexBIOSCI101Essential

Heterogametic sex = the sex that produces two

kinds of gametes and determines the sex of the

offspring.

Homogametic sex = the sex that produces oneiology

n o game e.


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Lecture6

Sex chromosomes in humans

BIOSCI101Essential

iology

Human karyotype: 46 chromosomes, 22 pairs of autosomes; sexchromosomes: male - heterogametic sex (XY), female -

homogametic sex (XX).

Lecture6

X Y sex determination in humans

BIOSCI101Essential

iology

Until the beginning of the 20th century, sex determination in humanswas ascribed to environmental factors ranging from nutrition to the

heat of passion during sex.


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Lecture6

What genes are on the human Y

chromosome

BIOSCI101Essential

iology

A British research team in 1990 identified a single gene, Sry (sexdetermining region of Y) which is responsible for maleness. SRY

triggers testicular development and is a transcription factor (DNA

binding protein).

Lecture6

The chromosomal basis of sex varies with

the organism

BIOSCI101Essential

iology


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Lecture6

2. Dosage compensation and X

chromosome inactivation.

Mary Lyon, a British geneticist in the 1960s

proposed that in females, each of the embryonic cellsBIOSCI101Essential

randomly inactivates one of the two X chromosomes.

The inactivated X chromosome is called a Barr

body.

Barr bodies are densely stained objects in the nucleiiology

o ema es. os arr o y genes are no

expressed.

Barr bodies are stable through mitosis but the

X is reactivated in the cells that give rise to ova.

Lecture6

Barr body

BIOSCI101Essential

iology

A barr body is a compact, darkly stained object which liesalong the inside of the nuclear membrane.


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Lecture6

Inactivation of the X involves modification of the DNA andhistones including the addition of methyl groups(CH3) to

the nucleotides.

X chromosome inactivation

BIOSCI101Essential

There are several genes located on the X chromosome

that are involved in the inactivation process.

One gene XIST (Xinactive specific transcript) becomes

active on the chromosome that will become the Barriology

body.

Multiple copies of the RNA product of the gene attach tothe X chromosome, interaction between the RNA and

chromosome initiates inactivation.

Lecture6

X inactivation & the tortoiseshell cat

BIOSCI101Essential

iology

Females consist of a mosaic of two types of cells: those with theactive X derived from the father and those with the active X derived

from the mother.


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Lecture6

3. Meiotic non-disjunction & alterations

of chromosome numberBIOSCI101Essential

Homologous chromosomes, or sister chromatids, fail

to separate during meiosis.

May occur in meiosis I - homologous pair dont

separateiology May occur in meiosis II - chromatids dont separate

Lecture6

Meiotic nondisjunction

BIOSCI101Essential

iology


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Lecture6

Abnormal chromosome number

Aneuploidy = abnormal number of certain

chromosomesBIOSCI101Essential

Trisomic = 1 extra chromosome (3 copies)

Monosomic = 1 less chromosome (1 copy)

Nullisomic = lack both chromosomes (0 copies)iology Polyploidy = more than two complete chromosome

sets e.g. triploid 3n and tetraploid 4n

Lecture6

4. Alterations to chromosome structure

BIOSCI101Essential

iology


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Lecture6

5. Human disorders due to chromosomal

alterations

BIOSCI101Essential

iology

Downs Syndrome - trisomy 21, incidence 1 in 700 children

born in the United States

Lecture6

Aneuploidy of human sex chromosomesBIOSCI101Essential

Nondisjunction of sex chromosomes produces a variety of

aneuploid conditions.

Most of these conditions appear to upset the genetic

balance less than conditions involving autosomes.

iology

This may be because the Y carries very few genes andbecause extra copies of the X are inactivated.


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Lecture6

Turners Syndrome - XO

BIOSCI101Essential

iology

Incidence 1 in 5,000, phenotypically female however are sterile astheir sex organs do not mature. If provided with oestrogen

replacement girls with Turners Syndrome do develop secondarysex characteristics.

Lecture6

Klinefelters Syndrome - XXY

BIOSCI101Essential

iology

Incidence 1 in 2,000 however some claim 1 in 500 males.

Individuals have male sex organs, are sterile and have abnormally

small testes. Even though the extra X is inactivated, some breast

enlargement & other female characteristics are common.

genetics lect 6 2011 colour 2 slides per page

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