Human Heredity

Chromosomes

• Every human cell has 46 chromosomes (23 pairs of chromosomes)

• A karyotype is a picture of the 23 pairs of chromosomes.

Examples of Karyotypes

Autosomes are the first 22 pairs of chromosomes (#1-#22)

Sex Chromosomes determine the sex and are pair #23

XX-female

XY-male

Female (XX)

Male (XY)

•Muskopf, Shannan. Online Images. The Biology Corner. 8 May 2007. http://www.biologycorner.com/bio1/celldivision-chromosomes.html

Pedigree Charts-shows genetic history of a family over many generations

•Muskopf, Shannan. Online Images. The Biology Corner. 8 May 2007. http://www.biologycorner.com/bio2/humangenetics.html

Generation I

Generation II

Generation III

Symbols in a Pedigree Chart

• Female

• Male

• Married Couple– horizontal line connects a

married couple

• Siblings

• Shaded: individual has the trait

• Not shaded: does not have the trait

• Half-shaded: carries the trait (heterozygous)

Pedigree Chart of a Dominant Trait• Huntingtons disease is a dominant disease that damages the

nervous system• One dominant allele means you get the disease • HH (homozygous) and Hh (heterozygous) will have the disease• hh (homozygous) will not have the disease• One parent has to have it to pass on to child

What is the genotype of the father?

What is the genotype of the son?

Pedigree Chart of a Dominant Trait

The father does not have the dominant disease so he must have 2 recessive alleles; his genotype is hh

The son has the dominant disease so he has to have at least one dominant allele. Is his genotype HH or Hh? One allele came from his father so his genotype is Hh.

Pedigree Chart of a Dominant Trait

hh HH

Hh

What are the chances that future offspring of this couple will have this dominant disease? Use the genotypes of the parents to draw a punnett square.

Pedigree Chart of a Dominant Trait

hh HH

Hh

All of the offspring have the dominant allele so 100% of their future children will have this dominant disease.

H H

h Hh Hh

h Hh Hh

Pedigree Chart of a Recessive Trait• Cystic fibrosis is a recessive disease causing digestive problems• HH (homozygous) and Hh (heterozygous) will NOT have the disease• hh (homozygous) will have the disease• Must inherit recessive allele from each parent to have the disease

What is the genotype of the father?

What is the genotype of the daughter?

Pedigree Chart of a Recessive Trait

She must have 2 recessive alleles to have this disease. Her genotype is cc.

The dad does not have this recessive disease so his genotype is either CC or Cc. Since he passed on one recessive allele to his daughter, his genotype is Cc.

Pedigree Chart of a Recessive Trait

What are the chances that future offspring of this couple will have this recessive disease? Use the genotypes of the parents to draw a punnett square.

Cc Cc

cc

Pedigree Chart of a Recessive Trait

Cc Cc

cc

C c

C CC Cc

c Cc cc

Only 1 out of 4 offspring have the recessive disease (cc). There is a 25% chance of future offspring having the disease.

Sex-Linked Traits (X-linked)

• Sex-linked genes are located on the sex chromosomes (23rd pair) and deal with the X and Y chromosome

• A female is XX; a male is XY• Sex-linked genes are shown as superscripts on

the X chromosome

Sex-Linked Traits in females

• If a disorder is caused by a recessive gene, a female’s genotype could be XBXB or XBXb and she will not have the disorder. The XBXb female would be a carrier.

• A female with a sex linked disorder would be written as XbXb because she must have both recessive alleles.

Sex-Linked Traits in males

• The Y chromosome does NOT carry a gene. A male without the recessive sex-linked disorder would be XBY. A male with the sex-linked disorder would be XbY.

• Sex-linked disorders are more common in males because males only need 1 copy of the recessive allele since they have only 1 X chromosome. Females must have 2 copies of the recessive allele since they have 2 X chromosomes.

• Common sex-linked disorders include color blindness and hemophilia (a bleeding disorder)

Sex-linked pedigree for color blindness

What is this male’s genotype?

Colorblindness is a recessive sex-linked disorder

This is a male so his sex chromosome is XY

The square is shaded so this male is colorblind

His genotype is XbY (Remember, there are no superscripts on the Y chromosome)

•Muskopf, Shannan. Online Images. The Biology Corner. 8 May 2007. http://www.biologycorner.com/bio4/notes/inheritance3.html

Sex-linked pedigree for color blindness

What is this male’s genotype?

This is a male so his sex chromosome is XY

The square is not shaded so this male has normal vision

His genotype is XBY

•Muskopf, Shannan. Online Images. The Biology Corner. 8 May 2007. http://www.biologycorner.com/bio4/notes/inheritance3.html

Sex-linked pedigree for color blindness

What is this female’s genotype?

This is a female so her sex chromosome is XX

This female is not colorblind so her genotype is either XBXB or XBXb

Her son is shaded so he is colorblind (XbY) and inherited the Y chromosome from his father and the Xb chromosome from his mother.

The mother has to have a Xb in her genotype

Her genotype is XBXb•Muskopf, Shannan. Online Images. The Biology Corner. 8 May 2007. http://www.biologycorner.com/bio4/notes/inheritance3.html

What percentages can be expected in the offspring of a cross between a female carrier for color blindness and a male with a normal color vision? Show your work with a punnett square.

Practice

What percentages can be expected in the offspring of a cross between a female carrier for color blindness and a male with a normal color vision? Color blindness is a recessive trait attached to the X chromosome. A female carrier means heterozygous.

female carrier XBXb

male with normal vision XBY

Practice

XB Xb

Y XBY XbY

XB XBXB XBXb

25% colorblind males

25% carrier females

25% normal males

25% normal females

• Karyotypes are used to determine if an offspring has the right number of chromosomes (46 in humans)

• Down Syndrome – 47 chromosomes because there are 3 copies of

chromosome #21– Causes mental retardation

3 copies of #21

•Muskopf, Shannan. Online Images. The Biology Corner. 8 May 2007. http://www.biologycorner.com/bio1/celldivision-chromosomes.html

• Amniocentesis-during pregnancy, a small amount of fluid from the sac surrounding the embryo is used to make a karyotype to determine if the fetus has the correct number of chromosomes

Diagnosis of Chromosomal Abnormality