Human Genetics

How many pairs of chromosomes are in each of our body cells?

A. 32B. 23C. 46D. 16

23 pairs! This is 46 total; we get 23 from mom & 23 from dad.

Body cells are also known as what?

A. GametesB. HistonesC. ChromatinD. Somatic Cells

Somatic Cells!

Which cells in our bodies do NOT have all 46 chromosomes?

A. GametesB. HistonesC. ChromatinD. Somatic Cells

Gametes! Gametes are also known as sex cells. THESE ARE SPERM & EGG CELLS!

How many chromosomes do human sperm or egg cells contain?

A. 32B. 23C. 46D. 16

23! They do NOT get 2 copies of each chromosome; they just get 1!

Gametes – Sperm & Egg Cells

• All gametes are haploid.

• In humans, that means each egg cell & each sperm cell has 1 copy of each chromosome (23 total chromosomes).– Egg Cells: all human egg cells carry 23 chromosomes, 1 of

which is a single X chromosome– Sperm Cells: in males, there are 2 types of sperm cells – 1

carries an X chromosome & 1 carries a Y chromosome

The 2 Types of Chromosomes

• Autosomes: non-sex chromosomes– Of the 46 chromosomes, 44 of them (22 pairs) are

autosomes.

• Sex Chromosomes: the last 2 chromosomes; they determine the sex of the person– Females have 2 X chromosomes (XX).– Males have 1 X and 1 Y chromosome (XY).

Is this person male or female?

When a sperm & egg cell combine, half the time the fertilized eggs (zygotes) are female & half of the time

they’re male.

FUN FACT!

There are actually slightly more men on Earth than women.

WHY? Shouldn’t it be half & half?

Since the Y chromosome is so much smaller, it’s lighter. That means that

the sperm cells that carry the Y chromosome can swim a little faster! This gives them an advantage & they can reach the egg cell more quickly!

Karyotypes – what’s that?

Karyotypes

• Scientists can analyze an individual’s chromosomes by taking a picture of cells during mitosis.

Why does this need to be done during mitosis?

• It’s easiest to view chromosomes during mitosis, because they are condensed!

Karyotypes

• From a picture of chromosomes, scientists can cut & paste to arrange the chromosomes in pairs to form a karyotype.

The last pair (#23) is the sex chromosomes.

A. Man B. Woman

Woman!There are 2 X

chromosomes, but no Y chromosome!

A. Man B. Woman

Man!It’s not labeled, but you can see that 1 of the sex chromosomes

is much smaller than the other (the X & the Y).

Why is karyotyping important?

Using Karyotypes• Genetic disorders can be identified by looking at an

individual’s karyotype.

Down Syndrome can be confirmed even before the baby is

born!

Human Chromosome Disorders

How often do they occur?

• Relatively high frequency in humans, but most embryos are spontaneously aborted– Developmental problems result from biochemical

imbalances.

About 60% of first trimester miscarriages are due to chromosome disorders.

Between 5-10% of stillborn deliveries are due to

chromosome disorders.

Human Chromosome Disorders

• Some conditions are survivable.

How do chromosome disorders occur?

Chromosome Abnormalities

• Incorrect number of chromosomes/mutations within the chromosome– Nondisjunction: chromosomes don’t separate properly

during meiosis– Chromosome Mutations: deletion, inversion, duplication,

translocation

Remember!

• During meiosis, a diploid cell (2 copies of each chromosome) divides & produces 4 haploid gametes (sperm or egg).– Gametes only have 1 copy of each chromosome!

Nondisjunction

• Sometimes, chromosomes may not separate properly during meiosis; this is called nondisjunction.

• If nondisjunction occurs, abnormal numbers of chromosomes (1 extra or 1 missing) are found in gametes & chromosomal disorders may result.

Nondisjunction

• Homologous chromosomes don’t separate properly during Meiosis 1.

OR

• Sister chromatids fail to separate during Meiosis 2.

Nondisjunction

• Trisomy: cells have 3 copies of a chromosome– In trisomies, the gamete

of one parent donated 2 of one type of chromosome to the child & the gamete of the other parent donated 1 (like normal).

Nondisjunction

• Monosomy: missing 1 chromosome– In monosomies, the gamete of one parent donated 1

chromosome & the other parent did not donate any.

Down Syndrome• Trisomy 21

– Every 1 in 700 children born in the US have Trisomy 21.

Chromosome 21 is the smallest, but an

abnormality still has severe effects.

Down Syndrome vs. Age of Mother

Mother’s ageIncidence of Down

Syndrome

Under 30 <1 in 1000

30 1 in 900

35 1 in 400

36 1 in 300

37 1 in 230

38 1 in 180

39 1 in 135

40 1 in 105

42 1 in 60

44 1 in 35

46 1 in 20

48 1 in 16

49 1 in 12

Women are born with all the egg cells they’ll ever have.

So, a 35 year old woman has 35 year old egg cells.

Other Examples of Disorders caused by Nondisjunction

• Klinefelter’s Syndrome – XXY (male)• Turner’s Syndrome – XO (female)

As long as there’s at least 1 Y chromosome, the karyotype is male!

Klinefelter’s Syndrome

• XXY– 1/2000 live births– Have male sex organs, but

are sterile– Feminine characteristics

• Some breast development

• Lack of facial hair– Tall– Normal intelligence

Turner’s Syndrome

• XO (1 X chromosome)– 1/5000 births– Varied degrees of effects– Webbed neck– Short stature– Sterile

Jacob’s Syndrome

• XYY– 1/1000 live births– Extra Y chromosome– Slightly taller than average– More active– Normal intelligence, slight

learning disabilities– Delayed emotional maturity– Normal sexual development Some men will never know

they have Jacob’s Syndrome!

Trisomy X

• XXX– 1/2000 live births– Produces healthy females!

• All but 1 X chromosome are inactivated.

Most mammals leave only 1 X chromosome activated in their cells. The inactivated chromosome(s) are

called barr bodies!

Remember! Chromosomal Mutations

• Error in replication:– Deletion: loss of a

chromosomal segment– Duplication: repeat a

segment

• Error in crossing over:– Inversion: reverses a

segment– Translocation: move

segment from 1 chromosome to another

How do we know if a growing embryo has a chromosome abnormality?

Genetic Testing!• Amniocentesis: a syringe takes a sample of the embryo cells,

then stains & photographs the chromosomes– Analysis of the karyotype

The Human Genome Project

• The genome of many organisms has been determined, including the human genome!

• Now, we are analyzing the code, finding genes coding for different traits, & discovering ways to isolate them & manipulate them.

• In gene therapy, an absent gene or faulty gene is replaced by a normal, working gene!

Video – Blind Dog Gene Therapy

Pedigrees – what are those?

Pedigrees

• Similar to a family tree; both are used to show relationships in a family

• Used to demonstrate how traits are passed from 1 generation to another

• Genetic counselors use pedigrees to follow how genetic disorders are inherited.

Pedigrees

• People who are heterozygous for a recessive genetic disorder (they are unaffected) are called carriers.– Sometimes, carriers of traits may be represented as a half-

shaded shape or a shape with a dot in the middle.

Pedigree Symbols

• Square: male• Circle: female• Shaded: affected individual

• Open: unaffected• Horizontal Line: marriage• Vertical Line & Bracket:

descendants of a marriage

female male

marriage parents children

Most genetic disorders are caused by mutations on autosomes (non-sex chromosomes).

• Autosomal Recessive Disorders: albinism, cystic fibrosis, tay-sachs disease

• Autosomal Dominant Disorders: achondroplasia (common form of dwarfism)

• Codominant Disorders

• Could this trait be inherited as a simple autosomal recessive?• Could this trait be inherited as a simple autosomal dominant?• Could this trait be inherited as a simple X-linked recessive?• Could this trait be inherited as a simple X-linked dominant?• Could this trait be inherited as a simple Y-linked trait?

• Could this trait be inherited as a simple autosomal recessive?• Could this trait be inherited as a simple autosomal dominant?• Could this trait be inherited as a simple X-linked recessive?• Could this trait be inherited as a simple X-linked dominant?• Could this trait be inherited as a simple Y-linked trait?

Draw the Pedigree!

• An unaffected man marries a woman who is a carrier for Duchenne Muscular Dystrophy, which is attributed to an X-linked gene.

• They have 4 children, 1 son with Duchenne, 1 carrier daughter and a daughter & son who are unaffected.

• The child with Duchenne Muscular Dystrophy dies in childhood.

• The carrier daughter marries and has 3 children of her own, 2 of which are carriers and 1 daughter of which is unaffected.