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Chromosomes and human inheritance. Genes are distributed in chromosomes. Each gene has a specific position (locus) on a chromosome. Each pair of homologous chromosomes has the same arrangement of genes. Genes on the same chromosome are linked. The genes may be identical or different.

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Presentation Transcript
slide1

Chromosomes and human inheritance

Genes are distributed in chromosomes. Each

gene has a specific position (locus) on a

chromosome.

Each pair of homologous chromosomes has

the same arrangement of genes. Genes on the

same chromosome are linked.

The genes may be identical or different.

Variant forms of genes are called alleles.

slide3

Alleles form through mutation (changes

in the DNA sequence)

There may be many different alleles within

the human population, but each individual

will have only two.

Combinations of alleles can be beneficial,

have no particular effect, or contribute

to disease

slide4

Humans have tens of thousands of genes

arranged on 46 chromosomes:

22 pairs of autosomes (homologous pairs)

1 pair of sex chromosomes

X and Y are NOT homologous

Genes located on “sex” (X or Y) chromosomes

are called sex- (or X or Y)-linked genes

slide5

The sex of an individual is determined by the

sperm:

Female gametes- X or X

Male gametes- X or Y

X X

XX XX

X

XY XY

Y

slide6

The Y chromosome is much smaller than the

X chromosome. For many years it was thought

that there were no genes on the Y chromosome.

It is now known that there are several genes

on the Y chromosome, that contribute to male

sexual development.

There are thousands of genes on the X

chromosome. Do females get a “double dose”

of these genes?

slide7

No- because of X inactivation

In each cell in a female, one of the X chromosomes

is condensed into a Barr body.

nucleus

Barr body

slide8

What happens if a zygote is formed with the

  • wrong number of chromosomes?
  • Aneuploidy- too many or too few chromosomes
    • Polyploidy- three or more sets of chromosomes
    • Triploidy- three sets of chromosomes
    • fairly common- 15-18% of miscarriages
    • A few are born alive- most die within
    • a month
    • Usually occurs when one egg is fertilized
    • by two sperm (75% of triploids)
    • In contrast, polyploid plants are common
slide9

Triploid infant

Approximately

1% of conceptions are

triploid

slide10

How does aneuploidy happen?

addition or deletion of an individual

chromosome

This usually happens through nondisjunction

(a chromosome pair fails to separate during

meiosis)

Trisomy 16 is the most common

Can be “mosaic” (some but not all cells are

affected)

slide11

n+1

Nondisjunction

n-1

n

n

slide12

When an aneuploid gamete fuses with a normal

gamete, the offspring will be aneuploid.

When this happens with an autosome the condition

is usually lethal. About 50% of all abnormalities

in fetal death involve trisomies.

A few survive.

slide13

Trisomy 13: Patau syndrome

most survive fewer than 6 months

defects in face, eyes, feet, nervous

system and heart

More common if maternal age is higher

(over 32). Also contributes to

Trisomy 18: Edwards syndrome

babies are very small, grow slowly,

usually female, and die within 2-4 months

due to heart or respiratory failure

Diagnosis by karyotyping

slide14

Trisomy 13 (Patau syndrome)

Cleft palate, eye and neurological problems

Average survival: 6 months

slide15

Edwards syndrome

Malformed hands and feet, heart defects

Life expectancy: 4 months

Maternal age is a factor

slide16

Trisomy 21 (Down syndrome)

occurs in about 1:900 live births

characteristic physical features

growth and mental development are usually

retarded

approximately 40% of all people with this

syndrome have heart defects

later in life, have very high incidence of

leukemia and/or Alzheimer’s disease

few live past age 50

slide18

Maternal age is a risk factor for trisomy

1:2000 births at age 20

1:100 at age 35

1:30 at age 45

Amniocentesis or CVS (chorionic villus sampling)

recommended for this reason

slide19

Aneuploidy of sex chromosomes

  • more common (1:400 in males, 1:650
  • in females
  • Turner syndrome (XO)
  • About 1:10,000 live births
  • females lack an X chromosome
  • short, broad-chested, do not undergo
  • secondary sexual development
  • infertile
  • not associated with mental retardation
slide21

II. Klinefelter syndrome (XXY)

about 1:1000 live births

not apparent until puberty

immature sexual development- occasion-

ally breast development

infertile

may have mild retardation

advanced maternal age increases risk

affected male may have several

X chromosomes- the more chromosomes,

the more severe the effect

slide23

III. XYY syndrome (Jacobs syndrome)

about 1:1000 live births

affected men tend to be unusually tall

MAY have personality disorders or

low intelligence (most don’t)

Once thought to make men predisposed to

criminal behavior (frequency of XYY men

is unusually high in prisons and mental

institution). No causal relationship has

been proven, though

slide24

Conclusions

At least one X chromosome is required for

survival (OY is lethal)

Extra X chromosomes seem to interfere with

normal development (even though

all but one is inactivated in cells)

The more X chromosomes are present, the

more severe the effect

slide25

Chromosomes can also be structurally altered

common (1:400 frequency)

Deletions- part of chromosome missing

Translocation- part of one chromosome

joined to another

associated with a hereditary form

of Down syndrome, some cancers

Duplications- part of chromosome is duplicated

effects vary

Chromosomal mutations

slide26

Recombination: portions of chromosomes

are rearranged (coy number variants)

Genes are moved “out of position”

inversions (ABCDACBD)

duplications (ABCDABBCD)

translocations (ABCDEFABCJKL

(GHIJKLGHIDEF)

Effects can be drastic (and permanent)

slide27

Cri du chat syndrome

Discovered by Jerome Lejeune

http://learn.genetics.utah.edu/content/disorders/whataregd/cdc/

rare chromosomal disorders
Rare chromosomal disorders

Robertsonian translocation and others

slide29

Uniparental disomy

herkules.oulu.fiherkules.oulu.fi

slide30

What kind of cell is affected and what

Are the consequences?

Germ-line cell (gamete)

these mutations are inherited

these give rise to alleles

Somatic cell

these affect the particular tissue

but cannot be inherited

example: tumors

Implications for gene therapy

slide31

Mutations- changes in gene sequence

damage

errors in copying gene sequence

Cells have mechanisms to repair this damage

but they are not always completely

successful

These changes lead to alleles

Sometimes they are harmless, sometimes they

lead to disease

slide32

How do mutations happen?

Spontaneous errors of replication

Chemicals (“mutagens”)

Radiation

Viruses

If damage to DNA is not repaired, the mutation

becomes incorporated into the genome.

slide33

Genetic disorders

Most are recessive (can be passed from two

healthy, “carrier” parents to a child)

Some alleles are more common than others;

some are more common in certain ethnic

groups than others

Sickle cell allele- African descent

Cystic fibrosis- European descent

Many of these alleles can now be identified through

testing

slide34

Genetic counseling

What contributes to a high-risk pregnancy?

Disease-causing alleles

dominant or recessive alleles-

relative risk of each?

Maternal age

(Paternal age not clear)

summary
Summary
  • Human chromosomes are analyzed by karyotyping
  • Study of variations in chromosomes structure can provide insight into health and disease
  • Major sources of variation are changes in chromosome number and arrangement
  • Polyploidy is much rarer in animals than plants
  • Aneuploidy is usually fatal
  • Structural changes can have drastic effects- or not