Chapter 7Extending Mendelian Genetics

Basic Patterns of Human Inheritance

Autosomal Recessive Genetic DisordersA recessive trait is expressed when the

individual is homozygous recessive for the trait.

Cystic Fibrosis

Affects the mucus-producing glands, digestive enzymes, and sweat glands

Chloride ions are not absorbed into the cells of a person with cystic fibrosis but are excreted in the sweat.

Without sufficient chloride ions in the cells, a thick mucus is secreted.

Video

Albinism

Caused by altered genes, resulting in the absence of the skin pigment melanin in hair and eyes

Tay-Sachs Disease

Caused by the absence of the enzymes responsible for breaking down fatty acids called gangliosides

Gangliosides accumulate in the brain, inflating brain nerve cells and causing mental deterioration.

Cystic Fibrosis Punnett Square

Basic Patterns of Human Inheritance

Autosomal Dominant Genetic Disorders

Huntington’s disease affects the nervous system.

Achondroplasia is a genetic condition that causes small body size and limbs that are comparatively short.

Huntington’s Disease Punnett Square

Pedigrees

A diagram that traces the inheritance of a particular trait through several generations

Inferring Genotypes Knowing physical traits can determine what

genes an individual is most likely to have.

Predicting Disorders Record keeping helps scientists use

pedigree analysis to study inheritance patterns, determine phenotypes, and ascertain genotypes.

Sex chromosomes determine an individual’s gender

Sex Determination

Sex-Linked Traits

Genes located on the X chromosome Red-green color blindness HemophiliaPunnett Square

Dosage Compensation The X chromosome carries a variety of genes

that are necessary for the development of both females and males.

The Y chromosome mainly has genes that relate to the development of male characteristics.

In females, one X chromosome is inactivated in each cell. The inactivated X chromosome is visible in stained cells as a Barr body.

X Chromosome Inactivation

Tutorial

Epistasis

Variety is the result of one allele hiding the effects of another allele.

An epistatic gene can interfere with the expression of other genes.

Albinism in mammals is caused by an epistatic gene that blocks the production of pigments.

Phenotype can depend on interactions of alleles.

Complex Patterns of Inheritance

Incomplete Dominance

The heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes.

Codominance

Both alleles are expressed in the heterozygous condition.

Sickle-cell disease is one example. People who are heterozygous for the trait

have both normal and sickle-shaped cells.

Multiple Alleles

Blood groups in humans

ABO blood groups have three forms of alleles.

Sometimes said to be codominant.

Multiple alleles can demonstrate a hierarchy of dominance.

Polygenic Traits

Polygenic traits arise from the interaction of multiple pairs of genes.

Read pg. 206 in textbook.

Environmental Influences

Environmental factors influence an organism’s phenotype

Diet and exerciseSunlight and waterTemperature

Gene Linkage

Read pgs.209-211

Karyotype Studies

Karyotype—micrograph in which the pairs of homologous chromosomes are arranged in decreasing size

Images of chromosomes stained during metaphase

Chromosomes are arranged in decreasing size to produce a micrograph.

Is this a normal karyotype?

Male or female?

Is this a normal karyotype?

Male or Female?

Is this a normal karyotype?

Male or Female?

Nondisjunction

Cell division during which sister chromatids fail to separate properly

Down syndrome, also called trisomy 21