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Patterns of Heredity & Human GeneticsChapter 12

+Pedigree

A pedigree chart can help trace the phenotypes and genotypes in a family to determine the chance that a child might have a certain genetic disorder.

A pedigree is made up of sets of symbols that identify males and females, individuals affected by the trait being studied, and family relationships.

+Pedigree Key

Female (unaffected)

Affected Female

Male (unaffected)

Affected Male

Female Carrier Male Carrier

+Pedigree Key

Mating (Parents)

SiblingsF1 Generation

Each horizontal row of circles and squares in a pedigree designates a generation,with the most recent generation shown at the bottom.

+Analyzing a Pedigree

See handout

+Why are pedigrees important? Most genetic disorders are caused by recessive alleles

and can be identified easily using a pedigree.

Cystic Fibrosis (CF) – Common among white Americans, there is a defective protein in the plasma membrane that causes thick mucus to build in the lungs and digestive tract. It is treated with physical therapy, special diets, and drug therapy.

Tay-Sachs disease – A recessive disorder of the central nervous system. Lipids fail to break down properly and accumulate in the cells of the CNS.

Phenylketonuria (PKU) – results from the absence of an enzyme that breaks down substances. It also causes a build up of by-products in the Central Nervous System.

+Why are pedigrees important?

Unlike the inheritance of recessive traits in which a recessive trait must be inherited from both parents for a person to show the recessive phenotype, many traits are inherited because of the dominant allele.

Tongue rolling, free-hanging earlobes, hitch-hiker’s thumb, etc. are all examples of dominant traits.

Huntington’s disease – a genetic disorder caused by a rare dominant allele. There are no effective treatments available and always results in death. The disease does not occur until the individual is between 30 and 50 years of age, and has almost always passed on the trait already.

Complete section 12.1 Review – Write the Questions & the Answers!

+Complex Patterns of Inheritance Mendel explained simple patterns of inheritance – if

you have the dominant trait it shows, and if you have two recessive traits, it shows! However, there are more complex patterns of inheritance than those studied by Mendel.

1. Incomplete Dominance

2. Codominance

3. Multiple Alleles

4. Sex-linked traits

5. Polygenic Inheritance

+Incomplete Dominance Incomplete dominance is demonstrated when the

heterozygous form of a gene is an intermediate between the homozygous dominant & the homozygous recessive.

Example RR (red snapdragon) and R’R’ (white snapdragon) crossed together result in RR’ (pink snapdragon)

+Codominance Codominant alleles cause the phenotypes of both

homozyotes to be produced in heterozygous individuals.

In codominance, both alleles are expressed equally.

For example, both flowers would be homozygotes with two different uppercase letters (R and W). The resulting offspring would show both red and white.

+Multiple Alleles

Although all of the traits you have studied thus far has only two alleles, it is common for more than two alleles to control a trait.

Traits controlled by more than two alleles is said to have multiple alleles.

For example, blood typing is controlled by three alleles: IA, IB, and i.

Both IA and IB produce a protein called an antigen on the surface of red blood cells. IA and IB are codominant.

Allele i is recessive and does not produce an antigen.

The four different phenotypes possible are shown on the next page.

+Blood Type [Multiple Alleles & Codominance]

Phenotype (Blood Type)

Possible Genotypes

A Antigen A IAIA or IAi

B Antigen B IBIB or IBi

AB Antigen A & Antigen B

IAIB

O No antigens ii

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+The Importance of Blood Typing Determining blood type is necessary before a person

can receive a blood transfusion because the red blood cells of incompatible blood types clump together, causing death.

Blood typing can also be helpful in solving issues parenthood – knowing the child’s blood type.

The father has type-O blood and the mother has type-A blood. Is it possible for their child to be born with type-AB?

+Sex-linked Traits We all know that a male’s sex chromosomes are XY and

a female’s sex chromosomes are XX.

When people talk about sex-linked genes, they are usually talking about genes on the X chromosome.

Because female’s have two X chromosomes, they have the ability to mask whatever recessive trait that is linked to one of their X chromosomes.

However, male’s only have one X and one Y and the Y is so much smaller than the X and carries far fewer genes. Males are more likely to display sex-linked traits because they are unable to mask it.

Examples: color-blindness and hemophelia (p. 333)

+Polygenic Inheritance Traits produced by two or more genes are called

polygenic traits. (Poly – Many // Genic – Genes)

For example: eye color, skin color

There are at least three genes that produce a range of eye color

All heterozygotes are intermediate in phenotype.

+Environmental Influences Phenotype is not determined only by genes. The

environment – the conditions surrounding an organism – also affect phenotype.

For example, the sex of sea turtles depends on the temperature of the environment in which the egg develops. Eggs in warmer nests result in females, and eggs in cooler nests result in males.

Likewise, genes and environment interact with human traits. Genes influence height, but do not completely control it. Take two identical twins and raise them in separate environments, one that has good nutrition and health care and the other that does not. Their height and size can vary greatly.

+KaryotypeA picture of all of the chromosomes

located in the nucleus of a eukaryotic cell

How can we see all the chromosomes? Chemicals are used to stain the

chromosomes. The stains produce a pattern of bands on the chromosomes. These patterns can be used to tell the different chromosomes apart.

+Why is it important to see all of the chromosomes?

Karyotypes allow scientists and doctors to see if there are any extra chromosomes or missing parts of chromosomes.

Each of these mutations leads to a genetic disorder. Down syndrome Klinefelter Syndrome Turner Syndrome

+Down Syndrome

A genetic disorder, Trisomy 21, in which a person has the presence of all or part of a third copy of chromosome 21.

It is typically associated with physical growth delays, characteristic facial features, and mild to moderate intellectual disability.

+Klinefelter Syndrome A genetic disorder characterized by an extra X

chromosome in males (genotype XXY, instead of XY)

It occurs in 1 in every 500-1000 male births, but the rate is up to 20 times higher in newborns with mental retardation rather than healthy birth

Klinefelter can result in behavioral problems, learning difficulties and speech disorder. If often goes undetected until the male is investigated for infertility.

Around 95-99% of KS men are infertile but sperm is found in 50% of them and can be injected directly into their female partner’s ovum.

+Turner Syndrome A genetic disorder that results when one normal X

chromosome is present in a female’s cells and the other sex chromosome is missing or structurally altered.

Turner syndrome is not genetically inherited, but occurs at random in 1 in every 2,000 women.

Many affected girls have an early loss of ovarian function. They do not usually go through puberty, unless they receive hormone therapy, and most are infertile.

Most women with TS have normal intelligence. Developmental delays, nonverbal learning disabilities, and behavioral problems are possible, although these characteristics vary among affected individuals.