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 crossed purebred plants with different characteristics and study the results

Genes and Dominance

• Mendel studied seven different pea plant traits• Trait- distinguishing characteristics that are

inherited, 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- piece of DNA that directs a cell to make a

certain protein• Each gene has a specific position on a pair of

homologous chromosomes

– The traits studied were controlled by one gene that occurred in two contrasting forms

– Alleles- different forms of a gene occurring at same location on the chromosome

Alleles

• Each parent donates one allele for every gene– Homozygous- alleles are

identical to each other- TT, tt

– Heterozygous- alleles are different from each other- Tt

• Genes influence the development of traits– Genome- all of an organism’s genetic material– Genotype- makeup of a specific set of genes– Phenotype- physical expression of a trait, physical

characteristics

• Some alleles are dominant, some are recessive

• Dominant allele- expressed as phenotype when at least one allele is dominant– Uppercase Letters

• Recessive allele- expressed as phenotype only when two copies are present– Lowercase letter

Segregation

• He allowed F₁ plants to self-pollinate – produced F₂ generation– F₁ generation, all plants had purple flowers– F₁ plants are all heterozygous– Some of the F ₂ generation, some plants had purple flowers some

had white

• Results of F₂- traits controlled by recessive allele represented one fourth of F₂ plants– Segregation- separation of alleles– Gametes- sex cells

• Law of Segregation– Organism inherit two copies of each gene, one from each parent– The two copies segregate during gamete formation

• 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

Punnett Squares• The inheritance of traits follows

the rules of probability• Punnett squares illustrate genetic

crosses– Punnett squares- grid system for

predicting all possible genotypes resulting from a cross• Axes represent the possible

gametes of each parent• The boxes show possible genotypes

of the offspring

• Punnett squares can be used to predict and compare the genetic variations that will result from a cross

Exploring Mendelian Genetics

• Dihybrid cross involves two traits• 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 YYRR) with plants that produced wrinkled green peas (genotype yyrr)– 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 he find out?– Mendel’s dihybrid

crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio

• What does this mean?– Law of independent assortment• Allele pairs separate independently of each other

during meiosis• 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

Traits and Probability

• Heredity patterns can be calculated with probability– Probability- likelihood that something will happen– Probability predicts an average number of

occurrences, not an exact number of occurrences– Probability applies to random events such as

meiosis and fertilization

Process of Meiosis

• How are new cells made?– Mitosis– Meiosis

• Meiosis- produces sex cells (gametes)– Reduces chromosome

number and creates genetic diversity

– Diploid cells undergo two cell divisions that result in 4 haploid cells

Meiosis

• You have two types of cells– Body cells- somatic cells– Germ cells- develop into gametes• Located in ovaries and testes• Gametes- sex cells: egg and sperm• Gametes have DNA that can be passed to offspring

• Gametes- have half the number of chromosomes that body cells have

Chromosomes and Meiosis

• How many chromosomes does a normal human have?– Body cells- 23 pairs; 46 total

• Homologous pairs of chromosomes have same structure

• 2 chromosomes- one inherited from mother and one inherited from father

• Autosomes- chromosome pairs 1-22

• Sex chromosomes- pair 23, X and Y

Chromosomes and Meiosis

• Body cells are diploid; gametes are haploid

• Diploid (2n) cells have two copies of every chromosome– Half the chromosomes

come from each parent

Chromosomes and Meiosis

• Haploid (n) cells have one copy of every chromosome– Gametes are haploid– Gametes have 22

autosomes and 1 sex chromosome

Chromosomes and Meiosis

• Meiosis makes haploid cells from diploid cells– Meiosis occurs in sex cells– Meiosis produces gametes

Process of Meiosis• 2 rounds of division in

meiosis– Meiosis I and meiosis II each

have four phases, similar to those in mitosis

– Pairs of homologous chromosomes separate in meiosis I

– Homologous chromosomes are similar but not identical

– Sister chromatids divide in meiosis II

– Sister chromatids are copies of the same chromosome

Meiosis I

Meiosis II

Meiosis and Genetic Variation

• Crossing over during meiosis increases genetic diversity– Crossing over is the exchange of chromosome segments

between homologous chromosomes– Occurs during prophase I of meiosis I– Results in new combination of genes

Meiosis and Genetic Variation

• Chromosomes contain many genes– Farther apart two genes

are located on a chromosome, the more likely they are to be separated by crossing over

– Genes located close together on a chromosome tend to be inherited together- Gene linkage

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

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 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