Human Genetics

Human GeneticsHuman Genetics

Review – What is a GENE?• A gene is the unit

that controls traits• Genes are passed

from parents to offspring

• Genes are located on our chromosomes

KARYOTYPE: A SET OF PHOTOGRAPHS OF CHROMOSOMES GROUPED IN ORDER OF PAIRS.

Karyotypes are used to detect chromosomal disorders and to determine the sex of an unborn

child

The 23rd pair of chromosomes are the sex chromosomes which determine an individual’s sex.

XX – Female or XY - Male

The first 44 chromosomes (22 pairs) are called autosomes.

At least 1 (X) chromosome MUST be present for survival

The presence OR absence of the (Y) chromosome determines sex of offspring

Normal Female Karyotype

This karyotype would be labeled as: 46,XX

Normal Male Karyotype

This karyotype would be labeled as: 46,XY

Chromosome Number Disorders

Nondisjunction - Failure of chromosomes to separate during meiosis.

– Abnormal offspring– Monosomy: one copy of a chromosome– Trisomy: three copies of a chromosome– Polyploidy: multiple copies of

chromosomes

Nondisjunction in Anaphase I

Trisomy 21 (Down Syndrome)

Turner’s Syndrome: 45XO

Klinefelter’s Syndrome: 47XXY

CHROMOSOMAL MUTATIONS

Deletion- piece breaks off and is lost.

Duplication - piece breaks off and attaches to a homologous chromosome

Translocation - piece breaks off and attaches to non-homologous chromosome.

Inversion - piece breaks off and reattaches upside-down.

What are Sex-Linked Traits? Trait determined by a gene on a sex chromosome (usually X chromosome).

Examples:Colorblindness, Hemophilia, Duchenne’s Muscular Dystrophy

• Females must receive 2 affected X’s in order to have the trait.

• Females with only 1 affected X are carriers.

• Males will always show a recessive trait located on the X chromosome because he only has 1 X.

Sex-Linked Disorders

• Genotypes for sex-linked traits are written using the X and Y chromosomes to show path of inheritance.

C = normal vision c = colorblindnessColorblind male = XcY

Colorblind female = XcXc

Carrier female = XCXc

**Remember males CANNOT be carriers**

Colorblindness

Sex-linked traits: Color blind father x carrier mother

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Sex-Linked Punnett Squares

• In sex-linked traits, probabilities for male and female offspring must be calculated separately because traits are inherited differently.

Sex-Linked Punnett Square Practice1. A colorblind female marries a man with normal vision.

What is the probability of them having colorblind children?

Key: ________________________________________

Cross: _______________________________________

Probability of having a colorblind daughter = ___________

Probability of having a colorblind son = ____________

Sex-Linked Punnett Square Practice2. A genetics counselor interviews a couple with a family history of

hemophilia to evaluate the possibility of having offspring with the disorder. The wife does not have hemophilia, but states that her father had the disorder. The husband is normal.

Key: ________________________________________

Cross: _______________________________________

Probability of having a daughter with hemophilia = ___________

Probability of having a son with hemophilia = ____________

Predictions and People

• What is a Pedigree - chart that shows how a trait and the genes that control it are inherited within a family.

How to Read a Pedigree

Circle = female

Square = male

Horizontal lines connect siblings or spouses

Vertical lines connect parents and children

Shaded figure = Individuals who are affected by the trait

Non-shaded = individuals who are unaffected

***Carriers are half shaded***

**When illustrated in a pedigree, children are placed in birth order from left to right (Oldest to youngest).

• What is a Carrier? - an individual who carries a recessive trait (heterozygous) that is not expressed and can pass on genetic disorders to offspring.

• Has normal phenotype

PracticeThe following pedigree shows the

inheritance of a recessive trait.

1. How many generations are shown?

2. How many children did Parents of Generation 1 have?

How many were boys?

How many were girls?

3. How many children did Parents of Generation II have?

How many were boys?

How many were girls?

4. Use the key to determine the genotypes of each person in this pedigree. (Hint: label all homozygous recessives first)

Key: R = Non-affected; r = affected

I Generation

II Generation

III Generation

1 2

3 4 5 6

7 8 9 10

Widow’s Peak is dominant over no widow’s peak

W = dominant

w = recessive Ww ww

Examples:

PKU (an inability to break down the amino acid Phenylalanine (can cause mental retardation)

Tay-Sachs (results in a nervous system breakdown and death)

Many genetic disorders caused by

AUTOSOMAL RECESSIVE GENES.

ALBINISM - a lack of pigment in hair, skin and eyes.

CYSTIC FIBROSIS - caused by a recessive allele on chromosome #7 - causes excess mucous in lungs and digestive tract, increased susceptibility to infections.

Tay-Sachs Recessive Trait Disorder

T = normal

t = tay-sachs

Tt Tt

Tttt Tt TT

Tt

Autosomal Dominant Disorders

• Dominant allele disorders are less common (typically a homozygous person (HH) dies therefore the trait can't carry on to offspring).

• Ex. Huntington Disease

Huntington’s Disease - dominance (H)

a. No symptoms until 30’s or 40’s

b. Nerve damage: loss of muscle control & mental function until death

Hh hh

Hh hhAffected Normal

Huntington’s Disease (dominant trait disorder)

H = affected w/disease

h = normalHh

hh

Hh

Hh HH

hh Hh