Human Inheritance Notes

Ch. 14 - Part II

Sex-Linked and Autosomal

Genetic Disorders

Karyotype Chart Once the cells have been gathered, scientists

will take a picture of the chromosomes and make a karyotype chart.

• Karyotypes show the 23 homologous pairs for the person in which the cells were taken.

• The pairs are put in order from longest to shortest and numbered from 1 to 23.

Pairs 1-22 are called autosomes.

Pair 23 are the sex chromosomes.

• Karyotype interactive

Is this a Male or Female Karyotype chart?

Female- XX

Down Syndrome

Chromosome number disorders link

• Cells contain one extra copy of chromosome # 21.

This results in: • characteristic facial features• short stature• mental retardation • shorter life-span• Increased risk for heart

problems, immune system problems, and cancer.

Down Syndrome link

Klinefelter Syndrome

This results in a male who has one extra X chromosome.47,XXY

This results in This male can not have

children. He can show mental

retardation. Is a true male, but can have

some female characteristics.

Klinefelter Syndrome Link

Sex-linked Inheritance (carried on the X chromosome)

Sex-linked disorders are more commonly seen in males.

Genes carried on the X chromosome are said to be sex-linked traits.

A carrier is a person who is heterozygous for a disorder, but does not show the disorder

Carriers of sex-linked traits are female.

Sex-linked Disorders (carried on the X chromosome)

Example 1 : Hemophilia a recessive allele written as Xh

What is hemophilia? “free bleeders”A clotting protein is missing.Uncontrolled bleeding can occur from

wounds and bruises.Affects 1 out of every 10,000 males

Phenotype Genotype

Normal Female XHXH

Carrier Female XHXh

Female with Hemophilia XhXh

Normal Male XHY

Male with Hemophilia XhY

Problem 1: A normal man marries a woman who is a carrier.

Create a Punnett square to determine the probability of passing on the allele to their offspring.

Xh

XH

YXH

XHY

XHXh

What are their chances of having a

daughter with hemophilia? 0%

a daughter who is a carrier? 25%

XhY a normal son? 25%a son with hemophilia? 25%

XHXH

Sex-linked Disorders (carried on the X chromosome)

Example 1 : Colorblindness a recessive allele written as Xc

What is the most common form of colorblindness?

Condition where the person cannot distinguish between the colors red and green.

Affects 1 out of every 10 males and 1 and 100 females.

Problem 1: A color-blind man marries a woman who is a carrier.

Create a Punnett square to determine the probability of passing on the allele to their offspring.

Xc

XC+

YXc

XC+YXC+Xc

XcXc

What are their chances of having a

daughter with normal vision 25%

a daughter who is a carrier? 25%

XcY a normal son? 25%a color-blind son? 25%

Is a sex-linked trait more often seen in males or females? Why?

Sex-linked traits are seen more often in males because males only have to inherit one copy of the trait on the X chromosome he received from his mom!

How can we tell if a genetic disorder is sex-linked?

Sex-linked Disorders (carried on the X chromosome)

Example 3 : Duchenne Muscular Dystrophy a recessive allele written as Xd

What is muscular dystrophy?A disorder that causes the muscles to

slowly weaken and eventually leads to death.

Affects 1 out of every 3,000 males

Autosomal DisordersThese are disorders carried on the autosomes, rather than the sex-chromosomes.

Most genetic disorders are recessive. Recessive disordered alleles produce

proteins that either malfunction or no longer function.

A heterozygous carrier has a normal phenotype because the one “normal” allele produces enough of the required protein.

Recessive Autosomal Disorders Albinism

Caused by a recessive allele on the chromosome 11.

Individuals who are homozygous recessive are unable to produce melanin, the pigment responsible for human skin color.

They have no pigment in their hair or skin, and are sensitive to light.

Recessive Autosomal Disorders Cystic Fibrosis

Caused by a recessive allele on chromosome 7.

Found in 1 in every 2,500 Caucasian births. 1 in 25 are carriers.

Individuals who are homozygous recessive make a defective cell membrane protein.

This creates extra mucus that clogs the lungs and breathing passageways leads to bacterial infections.

Two carriers have a 25% chance of having a child with cystic fibrosis.

Dominant Autosomal Disorders

Dominant disorders are typically lethal and are extremely rare.

This is because if a person inherits it he or she dies before they can have children.

Exceptions:

Huntington Disease• This caused by a rare dominant allele on

chromosome 4. 1/25,000 affected. • Symptoms start with a loss of muscle control. • *No symptoms until age 30 to 40. Mortality

within 15 years after systems first appear.• The gene for Huntington disease is dominant,

so that every child of a person with the disease has a 50% chance of inheriting the gene and having the disease.

• *This is a lethal disorder, homozygous dominant fetus, the baby miscarries.

Huntington Disease

• *This is a lethal disorder, which means if a person is homozygous dominant the baby miscarries.

• *No symptoms until age 30 to 40.

• *Why could not showing the disease until later in life be a problem? A person who doesn’t know they have the disorder can pass it on to their children before they show the disease.

Sickle Cell Anemia• (codominant disorder) It is found in 1

out of every 1,600 African Americans.• It is an anemia due to the sickled allele

producing an alternate from of hemoglobin (red blood protein), which causes cells to sickle or become crescent shaped.

• Individuals with two sickle cell alleles are said to have sickle cell anemia.

• It can lead to serious medical problems with sickled cells forming blood clots.

Pedigree ChartsThe family tree of

genetics What is a pedigree?

Constructing a pedigree

Interpreting a pedigree

What is a Pedigree Chart?

A Pedigree chart traces the inheritance of a particular trait through several generations.

One GOAL of using a pedigree chart is to figure out who are carriers of the trait, because this information is typically unknown.

Constructing a Pedigree

Male

Female

Married Couple-Horizontal Line

SiblingsVertical line

More than one Sibling:a horizontal line is drawn

with a vertical line coming down for each sibling.

Constructing a Pedigree

Fraternal twins-Two line branching from the

same point two eggs and two sperm cells.

Identical twins-Also called Maternal TwinsSame as fraternal twins but a

horizontal line is added.one egg and one sperm unite

and later splits to form two babies

Constructing a Pedigree

I

II

III

Constructing a Pedigree

Roman numerals to the left of the pedigree show the generations.

Birth Order: children are listed in birth order with oldest on left and youngest on the right.

Two horizontal lines from the same person represent two marriages / matings.

Example:This man first had a girl with the lady on the left, then had a boy and girl with the lady on the right.

Constructing a Pedigree

More Symbols in a Pedigree Chart

Full Shaded:Affected person who

shows a disorderHalf shaded:

Autosomal carrier Circle with dot:

X-linked carrier –always female

Deceased

Pedigrees are used to find out:who are carriers of the disorder &the probability of having a future child with

the disorder.To begin to interpret a pedigree, first

determine if the disorder is:Autosomal dominant Autosomal recessiveSex-linked (carried on the X chromosome)

Predicting using Pedigree Charts

Interpreting a Pedigree Chart

First ask: Is it a Sex-linked or Autosomal Disorder?

If there is a much larger number of males than females who are affected then the disorder is Sex-linked.

If there is a 50/50 ratio between males and females who are affected then the disorder is autosomal.

Interpreting a Pedigree Chart

If it is autosomal disorder then ask:Is it dominant or recessive?

If two parents do not show the trait and their children do show it, then it is an autosomal recessive disorder (parents are heterozygous)

If the disorder is autosomal dominant, then at least one of the parents must show the disorder.

Practice Examples

Does this pedigree show a Sex-linked or Autosomal disorder?

Answer:

Sex-Linked Disorder much larger number of males are affected

Practice Examples

Does this pedigree show a Sex-linked or Autosomal disorder?

Answer:

Autosomal Disorder 50/50 ratio between males and females