Human InheritanceHuman Inheritance

Review – What is Heredity?• Heredity is the

passing of traits from parents to offspring

• Genes are passed from parents to offspring (**Remember a gene is the unit that controls traits)

• Genes are located on our chromosomes

Autosomes vs Sex Chromosomes

• All human cells contain 46 chromosomes or 23 pairs of chromosomes

• The first 44 chromosomes (or 22 pairs) are called autosomes

• The 23rd pair of chromosomes are the sex chromosomes– In female the sex chromosomes are XX– In males the sex chromosomes are XY

Karyotypes• A karyotype is a photograph of chromosome

pairs.

• A normal human karyotype is the result of a haploid sperm containing 23 chromosomes fertilizing a haploid egg also carrying 23 chromosomes (23+23=46)

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

Normal Female Karyotype

This karyotype would be labeled as: 46,XX

Normal Male Karyotype

This karyotype would be labeled as: 46,XY

Pedigrees

• A pedigree is a diagram that follows the inheritance of a single gene through several generations of family

How to Read a Pedigree

Circle = female

Square = male

Horizontal line 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

**Children are place in birth order from left to right

PracticeThe following pedigree shows the

inheritance of a recessive trait.

1. How many generations are shown?

2. How many children did Parents 1-1 and 1-2 have?

How many were boys?

How many were girls?

3. How many children did Parents II-1 and II-2 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

Chromosome Number Disorders

• Number disorders occur because of a failure of the chromosomes to separate correctly during meiosis.

• Nondisjunction – chromosomes fail to separate– Nondisjunction in anaphase I: homologous pairs

fail to separate– Nondisjunction in anaphase II: sister chromatids

fail to separate

• In either case, gametes have an abnormal number of chromosomes

Nondisjunction in Anaphase I

Chromosome Number Disorders

Number disorders are NOT inherited; therefore they CANNOT be predicted

using Punnett squares

Autosomal Number Disorders

• Most autosomal number disorders are lethal

• The only autosomal number disorder that allows survival into adulthood is Down syndrome

Down Syndrome• Also known as Trisomy 21 because there

are 3 chromosomes in the 21st position

• Characteristics:– Flattened nose and face; upward slanting eyes– Growth, behavior and mental development are

all retarded– Risk of heart defects

• Incidence of having a baby with Down syndrome is higher in older mothers

Down Syndrome

Sex Chromosome Number Disorders

• Caused by nondisjunction of the chromosomes in the 23rd position or the sex chromosomes

• Turner Syndrome

• Klinefelter Syndrome

Turner Syndrome

• Also called 45,X0 because individuals lack a 2nd sex chromosome.

• Characteristics:– Female– Short stature– Underdeveloped sexually– Sterile (unable to bear children)– Normal life expectancy

Turner Syndrome

Klinefelter Syndrome

• Also called 47,XXY because individuals inherit an extra sex chromosome.

• Characteristics:– Symptoms appear at puberty– Males shows poor sexual development– Infertile– Normal life expectancy

Klinefelter Syndrome

Sex-Linked Inheritance• A gene is referred to as “sex-linked” if it is

located on a sex chromosome (either X or Y) • In humans, sex-linked genes are almost

always located on the larger X chromosome. • The Y chromosome is much smaller and

carries only a few genes related to sexual development.

• Females have 2 X chromosomes; males have 1 X.

Sex-Linked Inheritance

• Females will only show recessive traits located on the X chromosome if they are homozygous recessive.

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

• Females can be carriers for sex-linked recessive disorders. – A carrier someone who has the defective allele, but

doesn’t show the trait.

Sex-Linked Disorders

**Important – all of these disorders are sex-linked

RECESSIVE**

Sex-Linked Disorders1. Color Blindness: the inability to differentiate

and distinguish colorsEx: red-green colorblindness

Sex-Linked Disorders2. Hemophilia: Patients are missing an

enzyme required for normal blood clotting.– Have uncontrolled bleeding episodes– Treated with blood transfusions

Sex-Linked Disorders3. Duchenne’s Muscular Dystrophy

– Symptoms develop at 3-6 years of age.– Causes muscle weakening and the breakdown

of muscle fibers, leading to eventual death. – There is no available treatment or cure.

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

Carried female = XCXc

**Remember males CANNOT be carriers**

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 colorblind daughter = ___________

Probability of having a colorblind son = ____________

Autosomal Disorders• Most genes are carried

on the autosomes, the 44 chromosomes other than the sex chromosomes.

• Most genetic disorders are autosomal disorders.

• These disorders affect males and females equally.

Autosomal Disorders• Autosomal disorders can be divided into

three groups based on the pattern of inheritance.– Autosomal Recessive: Person needs two

recessive alleles to have the disorder– Autosomal Co-dominant: People with a

heterozygous genotype express similar symptoms to the abnormal phenotype

– Autosomal Dominant: the dominant allele will cause the disorder; only need one dominant allele

Autosomal Recessive Disorders1. Albinism: characterized by failure to produce

the pigment, melanin.– Lack coloration in skin, hair and eyes– At risk for skin cancer– Symptoms appear at birth

Autosomal Recessive Disorders2. Tay-Sachs Disease: characterized by the inability of

nerve cells to break down a specific type of lipid.– Symptoms appear between 3-6 months of age– Lipid build-up causes seizures, blindness, degeneration of

mental & motor skills– Death usually occurs at or before age 5– Historically associated with the Jewish population

Autosomal Recessive Disorders3. Cystic Fibrosis: characterized by excess

mucus production in the lungs & respiratory system– Symptoms appear just after birth– Suffer from frequent respiratory infections and

poor nutrition– Treatment helps patients live to their 20’s & 30’s

Cystic Fibrosis

Autosomal Recessive Disorders

4. Phenylketonuria or PKU: characterized by the inability to breakdown the amino acid, phenylalanine.– Build-up results in severe brain damage &

mental retardation– All babies born in US hospitals are tested for

PKU because it is easily treated with a diet low in phenylalanine

Phenylketonuria or PKU

Autosomal Co-Dominant Disorders1. Sickle Cell Anemia affects hemoglobin

production.– Hemoglobin is the protein that carries oxygen

in our red blood cells (RBCs)– Individuals that are homozygous for the

abnormal allele (ss) produce an abnormal hemoglobin which causes RBCs to “sickle” during periods of low oxygen• Ex: stress, high altitudes

– Sickled RBCs block vessels causing severe pain and brain damage

Sickle Cell Anemia

Autosomal Dominant Disorders1. Marfan Syndrome:

affects the connective tissue of the skeletal system, eyes and circulatory system

Characteristics:– Long limbs, vision

problem, susceptible to aortic rupture

Autosomal Dominant Disorders2. Huntington’s Disease: characterized by

deterioration of the nervous system

Characteristics:– Fatal disorder– Symptoms arise in the 40’s-50’s– Mental deterioration– Uncontrollable movements