Ch 11- Introduction to Genetics

• Genetics- scientific study of heredity• Gregor Mendel- father of genetics, laid the

foundation of the science of genetics– Used garden peas for his experiments

• He cross-breed true-breeding plants with different characteristics and study the results

Genes and Dominance

• Mendel studied seven different pea plant traits

• Trait- specific characteristic, such as seed color or plant height

• P (parental) generation-original pair of plants• F₁ (first filial) generation- offspring of original

pair of plants

• What did Mendel notice with the offspring?– All F₁ offspring had the characteristic of only one of the

parents• What did Mendel conclude about inheritance?– Traits are inherited through the passing of factors from

parents to offspring– Genes- chemical factors that determine traits– The traits studied were controlled by one gene that

occurred in two contrasting forms– Alleles- different forms of a gene

• What is the principle of dominance?– Some alleles are dominant and others are recessive– Allele for tall plants-dominant, allele for short-recessive

Segregation• He allowed F₁ plants to self-pollinate – produced F₂ generation• Results of F₂- traits controlled by recessive allele represented

one fourth of F₂ plants– Segregation- separation of alleles– Gametes- sex cells

• What happens during segregation?– The two alleles segregate from each other so that each gamete

carries only a single copy of each gene• Each F₁ plant produces two types of gametes, those with allele

for tallness and those with allele for shortness– Capital T represents a dominant allele, lowercase t represents

recessive allele. Result= F₂ generation with new combination of allele

Sec 2- Probability and Punnett Squares

• Probability- the likelihood that a particular event will occur– What is chance of that a coin lands heads up or

heads down?• How is coin flipping relevant to genetics?– The way in which alleles segregate is completely

random, like a coin flip• The principles of probability can be used to

predict the outcomes of genetic crosses

Punnett Squares• Punnett squares- diagram that shows gene combinations as a

result of genetic crosses• Punnett squares can be used to predict and compare the genetic

variations that will result from a cross• Homozygous- organisms that have two identical alleles for a

particular trait- TT or tt• Heterozygous- organisms that have two different alleles for the

same trait- Tt• Phenotype- physical characteristics

– All tall plants have same phenotype• Genotype- genetic makeup

– Genotype of one third of tall plants is TT while two thirds is Tt

Sec 3- Exploring Mendelian Genetics

• Does the gene that determines whether a seed is round or wrinkled in shaped have anything to do with the gene for seed color?

• Mendel’s crossed plants that produced only round yellow peas (genotype RRYY) with plants that produced wrinkled green peas (genotype rryy)– All offspring produced round yellow peas– Yellow and round are dominant over green and wrinkled

• Mendel allowed the F₁ generation to self-pollinate to produce an F₂ generation

• What did Mendel find out?– F₂ plants produced 556 seeds. 315 seeds were round and yellow,

32 were wrinkled and green. 209 seeds had combinations of phenotypes and alleles that weren’t found in parents

• What does this mean?– The alleles for seed shape segregated independently of those for

seed color– Genes for seed shape and seed color in pea plants do not

influence each other’s inheritance– Results-9:3:3:1 ratio -punnett squares predict this

• What is the principle of independent assortment?– Genes for different traits can segregate independently during the

formation of gametes

Dominant and Recessive Alleles• Some alleles are neither dominant nor recessive, and many traits are

controlled by multiple alleles or multiple genes• Incomplete dominance- one allele is not completely dominant over another

– Red plants (RR) crossed with white plants (WW)- pink (RW)• Codominance- both alleles contribute to the phenotype

– Black feathered chicken (BB) crossed with white (WW)- all speckled offspring (BBWW)

• Multiple alleles- many genes have more than two alleles – Human genes for blood type

• Polygenic traits- traits controlled by two or more genes – Variation in skin color

• Applying Mendel’s Principles- Thomas Hunt Morgan uses fruit flies to test Mendel’s principles– Produce large # of offspring

Sec 4- Meiosis• Where are genes located?

– On chromosomes in cell nucleus• Fruit fly, Drosophila, has 8 chromosomes

– 4 from male parent, 4 from female parent. These sets of chromosomes are homologous- each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parents

• Diploid- cell that contains both sets of homologous chromosomes– Represented by 2N– For Drosophila, the diploid number is 8, 2N=8– Diploid cells contain 2 complete sets of chromosomes and 2 complete sets of

genes• Haploid- gametes of sexually reproducing organisms, contain only single

set of chromosomes, only a single set of genes– For Drosophila, haploid number is 4, N=4

Phases of Meiosis

• Meiosis- process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell– 4 haploid cells are genetically different from one

another and from the original cell– Involves two distinct divisions- meiosis I and

meiosis II– Produces gametes

Meiosis I• Interphase, prophase, metaphase, anaphase, telophase• Interphase I- each chromosome is replicated• Prophase I- each chromosome pairs with its

corresponding homologous chromosome to form a tetrad– 4 chromatids in tetrad– Crossing over- chromosomes exchange portions of chromatids

• Produces new combinations of alleles

• Telophase I and Cytokinesis- Nuclear membranes form, cells separate into two cells

Meiosis II• After meiosis I, the two cells have sets of chromosomes and alleles

that are different from each other and from diploid cells• Prophase II- meiosis I results in two haploid daughter cells, each

with half the number of chromosomes as original cell• Metaphase II- chromosomes line up in a similar way to the

metaphase stage of mitosis• Anaphase II- sister chromatids separate and move toward opposite

ends of the cell• Telophase II and Cytokinesis- meiosis II results in four haploid

daughter cells• Four daughter cells contain just 2 chromosomes each• Meiosis

Gamete Formation

• In males, meiosis results in four equal sized gametes called sperm

• In females, only one large egg cell results from meiosis– Other three cells called polar bodies are not involved in

reproduction• How is mitosis different than meiosis?– Mitosis results in the production of two genetically

identical diploid cells, whereas meiosis produces four genetically different haploid cells

• Comparison of Meiosis and Mitosis

Sec 5- Linkage and Gene Maps

• Thomas Hunt Morgan- researched fruit flies in 1910 and discovered that chromosomes assort independently, not individual genes– Each chromosome is actually a group of linked genes

• How did Mendel manage to miss gene linkage?– Six of the seven genes he studied are on different

chromosomes, the two genes found on same chromosome are so far apart they assort independently

Gene Maps• If two genes are found on the same chromosome, does this

mean that they are linked together forever?– Crossing over during meiosis separates genes on same chromosomes– What does this lead to?

• Genetic diversity

• Alfred Sturtevant- created gene map showing the relative locations of each known gene on one of the Drosophila chromosomes– He showed genes close to each other on chromosome are usually

inherited together– His method has been used to construct genetic maps- including maps

of human genome