Genetics Chapter 9 – Patterns of Inheritance. I. A Historic Overview Primitive civilizations -- domestication of plants and animals, important demonstration.

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

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

• Mendel’s Experiments:– He performed controlled breeding experiments– Pea plants have distinct characteristics that are

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

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

Mendel’s Contributions – 3 Principles

A. The Principle of Dominance

• Mendel said: Some factors for a trait show up when present (dominant). Other factors are overpowered by the dominant and show up only when the dominant factor is not present (recessive)

• Today: The factors are alleles of the same gene• Correction: Many traits do not follow

dominant and recessive inheritance. These are called non-Mendelian traits.

B. The Principle of Segregation• Mendel said: Factors for the SAME trait are

separated from each other during the formation gametes and get rearranged after fertilization.

• Today: Alleles for the same gene get separated during meiosis (haploid cells form) and get paired up again during fertilization.

• Correction: None

C. The Principle of Independent Assortment

• Mendel said: Factors for DIFFERENT traits separate from each other during the formation of gametes and get rearranged again during fertilization.

• Today: Genes for different traits separate from each other during meiosis and get rearranged again during fertilization.

• Correction: It is chromosomes not genes that separate, so if two genes are located on the same chromosomes, they do not assort independently, they are said to be linked genes and inherited together.

• Solving monohybrid problems with dominant and recessive inheritance – we will have many practice problems

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

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

• In many organisms, we can use random crosses between organisms.

• Test cross – a cross that is used to determine the genotype of an unknown dominant organism by crossing it with a homozygous recessive organisms.

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 deterioration, uncontrollable

movements

Variations to Mendel’s Principles• 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)

• Lethal alleles -- some genotypic combinations are deadly and will not be counted in the live offspring ratio.

• Many practice problems will be given

• 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)

• The study of changes in inheritance that is not caused by changes in the DNA is called epigenetics – genes can be activated or inactivated, chromosomes can be tightly packed or loosened.

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

• Most of the chromosomes are autosomes, they are the same in males and females.

• Traits that are located on the autosomes follow autosomal inheritance.

• 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.

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.

Chi-Square Test• This test can be used for many different

biological principles to test if a given hypothesis (called null hypothesis) is supported by the collected data or not.

• Null hypothesis – a hypothesis that states that the examined independent variable does not influence the dependent variable.

• Alternative hypothesis is set if the variables are influenced by each other.

• The steps of performing a Chi-square test:– State a null hypothesis (and an alternative hypothesis

– this is not always necessary, just follow the instructions)

– Calculate the expected value– Calculate the χ2

value by using the equation (no need to memorize it, you will have this given):

– Determine the degrees of freedom by using categories – 1 = degrees of freedom

• Determine the critical value at 0.05 probability by using the critical value table.

• If the critical value is larger than the Chi-square value, your null hypothesis is accepted, if the critical value is lower than the Chi-square value, the null hypothesis is rejected.

• We will have many practice problems.