GENETICS Chapter 9 – Patterns of Inheritance

I. A Historic Overview Primitive civilizations -- domestication of

plants and animals, important demonstration of early genetic engineering, lead to agricultural development

Gregor Mendel -- laid down the foundation for the field of genetics (early 1800s) (http://science.discovery.com/videos/100-greatest-discoveries-shorts-genetics.html )

Morgan (1900s) – used fruit flies to identify chromosomes as region of cell where genes are stored in the cell

Modern Genetics Populations Genetics - Evolution Oncology, oncogenes and Cancer Genetic Disease and Gene Therapy Recombinant Technology (e.g., crop

resistance, animal breeding, etc...) http://www.youtube.com/watch?v=YXPnQvcqHkg

DNA Fingerprinting

II. Mendelian Inheritance and Key Terms

Genetics – the study of inheritance (the transmission of traits from one generation to the next)

Mendel’s Experiments: He performed controlled breading

experiments Pea plants have distinct characteristics that

are passed on from one generation to the next in determined mathematical ratios

Traits: (see picture)

He experimented on peas with monohybrid crosses (following the inheritance of one single trait when two heterozygous parents are crossed).

Mendel’s contributions Different morphological traits come in two's (e.g.,

smooth or wrinkled seed), must be 2 particles inside the cell that determine the morphological trait, (Today we know theses are alleles = alternative forms of a gene)

Relationships exist between alleles, most common is dominance (an allele that is more powerful than the other allele of the same gene). Recessive alleles are masked by the dominant ones

Law of segregation - alleles segregate on gametes (today we know – because the gametes are haploid, they carry only one copy of each gene)

Law of independent assortment – during gamete formation (meiosis), alleles of DIFFERENT traits are arranged independently from one another

Solving monohybrid problems with dominant and recessive inheritance

Solving dihybrid cross problems (crossing two traits at a time where the parents are heterozygous to both traits)

III. How Can We Study Human Genetics?

To study human inheritance, human pedigrees are used – a chart to follow a certain trait over several human generations.

You must be able to determine the type of inheritance by using human pedigrees

IV. Examples of Human Traits Controlled by a Single Gene

Recessive disorders: Albinism – lack of pigment in skin, hair and eyes Cystic fibrosis – excess mucus in lungs, digestive tract

and liver Tay-Sach’s disease Sickle-cell disease – sickled blood cells, damage to

many tissues (Pamela’s story: http://www.nhs.uk/Conditions/Sickle-cell-anaemia/Pages/Introduction.aspx The disease: http://www.youtube.com/watch?v=9UpwV1tdxcs )

Dominant disorders: One type of Alzheimer’s disease – mental

deterioration Huntington’s disease – mental deteioration,

uncontrollable movements

V. Variations to Mendel’s Laws More often the inheritance patterns are more

complex than simple dominant and recessive inheritance.

Incomplete dominance – a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype. (ex.: red and white snapdragons will have pink flowered offspring)

Codominance – It occurs when both of the contributions of both alleles are visible and do not over power each other in the phenotype (ex.: A and B blood groups)

Pleiotropy -- occurs when a single gene influences multiple phenotypic traits (ex. Sickle cell disease)

Polygenic inheritance – A simple phenotypic characteristic is inherited by the interaction of at least two genes. (Ex. Skin color in humans)

The frequency of the traits with polygenic inheritance follow the shape of a bell curve.

Many characteristics result from the combination of heredity and environment (skin color, weight, height)

VI. Chromosome Theory of Inheritance

Genes occupy specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment during meiosis.

Because of the chromosomal theory, if genes are located on the same chromosome, they are inherited together and not independently from one another – linked genes

Chromosomes that are responsible for the determination of the gender – sex chromosomes

In humans and most mammals XX determines a female and XY determines a male.

In other organisms there may be a different system of sex chromosomes.

VII. Sex Linkage Genes that are located on either the X or

Y chromosomes are sex linked These genes are inherited differently in

males and females because the X and Y chromosomes do not carry the same genes.

Genetic disorders that have genes on the X chromosome show up more frequently in males than females. While Y-linked disorders only show up in males.

Males get their X chromosome from their mother.

You must be able to solve genetic problems with sex-linked inheritance in traditional genetic problems and in pedigrees.