Genetics

• Genetics is the study of heredity, which is the passing of traits from parents to offspring

• Mendel was successful in determining the basic laws of heredity because of his experimental method

• Mendel`s approach was mathematical, using many repeated trials to find the likely outcome from a large sample size

• Mendel choose to study garden peas, Pisum sativa, which has many useful features– many traits with 2 distinct forms (no intermediates) – mating can be controlled, using either self-fertilization or cross-

fertilization – small, grows easily, matures quickly, produces many offspring

Mendel observed that traits are expressed in simple ratios

• 1. Monohybrid crosses involve one pair of contrasting traits

Step 1: use self-fertilization for several generations to ensure that parental generations are true-breeding

Step 2: cross-fertilize two contrasting P generation plants, with offspring known as the F1 generation (1st filial generation)

Step 3: allow F1 generation to self-fertilize, producing the F2 generation (2nd filial generation)

• 2. Mendel`s results • F1 generation always shows one of the two

contrasting traits, known as the dominant trait (recessive not seen)

• F2 generation always shows both traits, but in a 3:1 ratio of dominant to recessive

• 3:1 ratio of dominant:recessive was constant for seven different pairs of contrasting traits studied by Mendel

Mendel's Work Became a Theory of Heredity

• Mendel's Hypotheses – For each inherited trait, an individual has two copies of the gene,

one from each parent – There are alternative versions of genes, known as alleles – When two different alleles occur together, one of them may be

completely expressed, while the other may have no observable effect on the organism's appearance

• 1) dominant trait expressed • 2) recessive trait not expressed

When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. When gametes unite during fertilization, each gamete contributes one allele.

• 2. Mendel`s findings in modern terms a. homozygous refers to two of the same alleles of a particular gene in an individual

• homozygous dominant AA

• homozygous recessive aa

• heterozygous refers to two different alleles of a particular gene in an individual: Aa

– genotype refers to the set of alleles in an individual

– phenotype refers to the physical appearance of a trait in an individual

Mendel's Ideas Gave Rise to the Laws of Heredity

1. law of segregation: the two alleles for a trait segregate when gametes are formed during meiosis

2. law of independent assortment: the alleles of different genes separate independently of one another during gamete formation

The Human Genome

• Defn.: the entire collection of genetic material in each typical cell of the human body

• Human genome includes 46 individual nuclear chromosomes and one mitochondirial chromosome

• Contains ~30,000 genes (about 200 are bacterial in origin)

• Analysis of genome called genomics

Study of Protein

• Proteomics: analysis of protein codes in order to understand the role of each individual one.

• Proteome: entire protein group in human genome

Ideogram

• Cartoon of a chromosome

• A banding pattern is the characteristic pattern of dark and light bands that appears when a chromosome is stained with a chemical solution and then viewed under a microscope. These bands are used to describe the location of genes on each chromosome.

• Short arm called the p-arm

• Longer arm is the q-arm

Meiosis

• Remember meiosis causes gametogenesis

• Each gamete is a haploid (Mendels principle os segregation)

• Make sure you review steps of meiosis

Independent assortment

• Each chromosome separates itself independently in meiosis

• Maternal and paternal chromosomes get mixed up and redistributed

• Each gamete becomes genetically unique: called genetic variation

• Gene linkage: genes on an individual chromosome stay together

• Exception---crossing over– During meiosis homologous chromosomes

line up along the equator and exchange genes

Karyotype

• A picture of homologous chromosomes

• Allows to see – Trisomy (extra chromosome)

– Monosomy (missing chromosome)

– Broken chromosomes

Punnett Squares Can Predict the Expected Results in Crosses

a. write the parental genotypes (e.g., Aa x Aa)

b. write the possible gametes from one parent along the top of the square (using the law of segregation: Aa --> A, a)

c. write the possible gametes from the other parent along the left side of the square (using the law of segregation: Aa --> A, a)

d. write the combination of two alleles produced during fertilization by combining the allele along the top of the box with the allele along the side of the box

e. calculate the genotypic ratio

f. calculate the phenotypic ratio

• 2. Dihybrid crosses: crosses the involve two traits

• 3. Test crosses: an individual whose phenotype is dominant, but whose genotype is unknown, is crossed with a homozygous recessive individual

Probabilities Can Also Predict the Expected Results of Crosses

• 1. Probability is the likelihood that a specific event will occur, expressed as a decimal, fraction or percentage

• 2. Probability of a specific allele in a gamete a. if heterozygous, 50% b. if homozygous, 100%

• 3. Probability of the outcome of a cross: the probability that a combination of two independent events will occur is the product of the two independent probabilities

Family Pedigrees Can Be Used to Study How Traits Are Inherited

• 1. A pedigree is a family history that shows how a trait is inherited over generations, especially used for genetic disorders

• 2. Autosomal or sex-linked? – a. if autosomal, the trait appears in both sexes equally

– b. if sex-linked, the trait is on the X chromosome, and appears more often in males, with only one X chromosome, than females, with 2 X chromosomes

• 3. Dominant or recessive?– a. if autosomal dominant, the an individual with the trait must have a parent with

the trait

– b. if autosomal recessive, the individual with the trait might have both parents without the trait, if both are heterozygous carriers

• 4. Heterozygous or homozygous? – a. for autosomal traits, the dominant phenotype will be the same for

homozygous dominant and heterozygous individuals

– b. for autosomal traits, the recessive phenotype will only be seen in homozygous recessive individuals

Most Traits are not Controlled by Simple Dominant-Recessive Alleles • 1. Polygenic traits are produced by several genes influencing a trait

together – a. crossing over and independent assortment produce many combinations– b. thus, many intermediate conditions (hair color, eye color, skin color)

• 2. Incomplete dominance is produced when neither allele is dominant, producing a trait intermediate between the two alleles (curly x straight=wavy)

• 3. Codominance involves two dominant alleles that are both expressed in the heterozygous genotype (type AB blood)

• 4. Multiple alleles involve genes with three or more alleles (A, B, AB, O blood)

• 5. Traits influenced by the environment

– a. human height influenced by nutrition – b. arctic fox coat color influenced by temperature– c. hydrangea flower color influenced by soil acidity d. human skin color

influenced by exposure to sun

Some Traits are Caused by Mutations

• 1. Sickle cell anemia is an autosomal recessive disorder producing defective hemoglobin a. usually fatal in a homozygous genotype, thus

selected against

b. for heterozygotes, protects from malaria, thus selected for

c. combination of two selective pressures balance each other

• 2. Hemophilia is a sex-linked recessive disorder impairing blood clotting

• 3. Huntington's disease (HD) is an autosomal

dominant disorder producing progressive degeneration of some nervous system functions

• 4. Detecting and treating genetic disorders

– a. genetic counseling – b. gene technology, such as gene therapy