Introduction to Genetics

Chapter 11

Warm Up

• Please complete the pretest that you picked up as you came in!

The Work of Gregor Mendel

• • Genetics- the study of heredity.

Gregor Mendel- Austrian monk- the father of genetics- carried out his work on garden peas.

– Pea flowers are naturally self-pollinating, which means that sperm cells fertilize the egg cells in the same flower, thus the new flower inherits all of their characteristics from the single plant that bore them.

The Work of Gregor Mendel

• True-Breeding- organisms that produce offspring identical to themselves if allowed to self pollinate.

– ie: one stock of seeds would produce only tall plants, another only short plants. One stock would produce yellow seeds, another only green seeds.

The Work of Gregor Mendel

• Cross-Pollination- produced seeds that had two different plants as parents.

Genes and Dominance

• • • Trait- a specific characteristic that varies from one individual to another.

Each original pair of plants is called the P (parental) generation.

The offspring are called the F 1 (first filial) generation.

Genes and Dominance

• • • Hybrids- the offspring of crosses with different traits.

Genes- chemical factors that determine an organism’s traits. Genes are passed from parents to their offspring (one gene from each parent).

Alleles- different forms of a gene. – ie: forms that produce tall vs. short plants or round vs. wrinkled seeds.

Genes and Dominance

• Principle of Dominance- some alleles are dominant and others are recessive.

– An organism with a dominant allele for a specific form of a trait will always exhibit that form of the trait.

– An organism with a recessive allele for a specific form of a trait will exhibit that form only when the dominant allele for the trait is not present.

Segregation

Segregation

• • The reappearance of the recessive trait indicated that at some point the paired alleles are separated or go through segregation during the formation of gametes.

Gametes- sex cells

Exit Slip

• Explain the difference between true-breeding and hybrid plants.

Warm Up Exercise

• • What are dominant and recessive alleles? How do we represent them?

What are gametes?

Genetics and Probability

• Probability- the likelihood that a particular event will occur.

– Remember, past outcomes do not affect future outcomes.

– The principles of probability can be used to predict the outcomes of genetic crosses.

Punnett Squares

• Punnett Square- a diagram that determines what gene combinations can result from a specific genetic cross.

Punnett Squares

• • • • Homozygous- organisms that have two identical alleles for a particular trait (TT or tt).

– Homozygous organisms are true-breeding for a particular trait.

Heterozygous- organisms that have two different alleles for the same trait.

Phenotype- physical characteristics.

Genotype- genetic makeup. – Homozygous dominant, heterozygous, homozygous recessive

Probability

• Probabilities predict averages. Thus, the larger the number of offspring, the closer the resulting numbers will be the expected values.

Exit Slip

• An F 1 plant that is homozygous for shortness is crossed with a heterozygous F 1 plant. What is the probability that a seed from the cross will produce a tall plant? Use a punnett square to explain your answer.

Warm Up Exercise

-Determine what maternal genotype(s) would most likely yield offspring with such characteristics. -Use the genotype that you came up with for the mother to complete a punnett square.

The Two Factor Cross

The Two Factor Cross

Independent Assortment

• Independent Assortment- genes for different traits segregate independently, such that the genes for one trait do not influence another trait.

Beyond Dominant and Recessive

• Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete Dominance

• Incomplete Dominance one allele is not completely dominant over another. – In incomplete dominance, the heterozygous phenotype is somewhere in between the two homozygous phenotypes.

Codominance

• Codominance- both alleles contribute to the phenotype. – Ex: AB blood type

Multiple Alleles

• Multiple Alleles- genes having more than two alleles.

– This does not mean that an individual can have more than two alleles, it means that more than two possible alleles exist in a population for a given trait.

– Ex: human blood type

Hair Color and Multiple Alleles

• FIGURE 11.12

Polygenic Traits

• Polygenic Traits- controlled by two or more genes.

– Ex: skin color of humans- controlled by more than four different genes.

Applying Mendel’s Principles

• • • • Mendel’s principles don’t apply only to plants.

In the early 1900s, Thomas Hunt Morgan found a model organism to advance the study of genetics, the common fruit fly, Drosophila melanogaster.

Fruit flies were an ideal organism for several reasons:

–

They could produce plenty of offspring, and they did so quickly

Morgan and other biologists learned that Mendel’s principles applied not to just pea plants, but other organisms and humans too.

Genetics and the Environment

•

The characteristics of any organism are not determined solely by the genes it inherits, but by the interaction between genes and the environment.

– Ex: genes may affect a sunflowers height and the color of its flowers, but these same characteristics are also influenced by climate, soil conditions, and availability of water.

Warm Up Exercise

• In rabbits, black hair is due to a dominant gene B, and brown to its recessive allele b. Short hair is due to the dominant gene S and long hair to its allele s. In a cross between homozygous black, long hair individual with a homozygous short, brown hair individual, what would be the nature of the F1 generation?

Chromosome Number

• • The Chromosomal Theory of Inheritance genes are located in specific positions on chromosomes.

Homologous- chromosomes form in pairs, one from the male parent and one from the female parent.

Chromosome Number

• • Diploid- a cell that contains both sets of homologous chromosomes. (2N) – Diploid cells contain two complete sets of chromosomes and two complete sets of genes.

Haploid- a cell only containing one set of chromosomes. (N)

Meiosis

• Meiosis- a process of reduction division in which the number of chromosomes is cut in half through separation of homologous chromosomes in a diploid cell.

– Meiosis takes place in two distinct divisions: Meiosis I and Meiosis II.

Meiosis

• • Interphase- cells undergo DNA replication, forming duplicate chromosomes. Nucleus breaks down.

Meiosis I

– Prophase I- each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Crossing over occurs in prophase I.

– Metaphase I- chromosomes line up in the middle of the cell and attach to spindle fibers.

– Anaphase I- spindle fibers pull chromosomes toward opposite ends of the cell.

– Telophase I and Cytokinesis- nuclear membrane reforms and the cell divides into two cells.

Meiosis I

• Crossing Over- in prophase I, homologous chromosomes exchange portions of their chromatids. – This produces new combinations of alleles and allows for more genetic variation.

Meiosis I

Meiosis

•

Meiosis II

– Prophase II- meiosis I resulted in two haploid daughter cells with half the number of chromosomes as the original cell.

– Metaphase II- the chromosomes line up in the middle of the cell.

– Anaphase II- sister chromatids are separated and move toward opposite ends of the cell.

– Telophase II and Cytokinesis- nuclear membranes form and meiosis II results in four haploid daughter cells.

Meiosis II

Gamete Formation

• In male animals, meiosis results in four equal-sized gametes called sperm.

Gamete Formation

• In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.

Comparing Mitosis and Meiosis

• Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells.

Exit Slip

• In human cells, 2N = 46. How many chromosomes would you expect to find in a sperm cell? In an egg cell? In a white blood cell?

Gene Linkage

• • Thomas Hunt Morgan studied gene linkage on fruit flies. His conclusions: – Each chromosome is actually a group of linked genes.

– Mendel’s principle of independent assortment still holds true.

It is the chromosomes that assort independently, not the individual genes.

Gene Maps

•

Crossing-over during meiosis sometimes separates genes that had been on the same chromosomes onto homologous

chromosomes.

– Crossover events occasionally separate and exchange linked genes and produce new combinations of alleles, which helps generate genetic diversity.

Gene Maps

• Alfred Sturtevant (student to Morgan) – The further apart two genes are, the more likely they are to be separated by a crossover.

– This allowed him to use recombination frequencies to determine the distances between genes.

Gene Maps

• Gene Map- shows the relative location of genes on a chromosome.

– If two genes are close together, the recombination frequency between them should be low, since crossovers are rare. – If they are far apart, recombination rates between them should be high.

Gene Map of Drosophila

Warm Up Exercise

• If two genes are on the same chromosome but usually assort independently, what does that tell you about how close together they are?

Human Chromosomes

• Karyotypes- a picture of chromosomes grouped together in pairs.

– Humans have 46 chromosomes. Two of them are sex chromosomes, because they determine an individual’s sex. The remaining 44 chromosomes are called autosomes.

– Females are XX and males are XY.

Human Traits

• Pedigree Charts- shows the relationships within a family.

Ed Smith James S.

Jones Anna Arpa Duvall Jones 46 John Jack Woosley 60 Roseman Jones Woosley Effie Jane Hayes Austin William Austin Verda Morehead 88 Alvin Lindsey 86 Ann Arpisine Woosley Lindsey 77 Joseph L.

Woosley 66 Marth E.

Pressley Woosley Tom Grundy John Brackett Archie Castle Huckleberry Brackett James Potter Thomas Roy Brackett Anna Loraine Brackett Thomas Pearl Brackett Potter Dame 79 Delores Lee Dame Ferrell Emuel Smith Mable Crabtree Ura Austin Brackett Clint Austin Bernice Austin Grimmizon Mittie Eva John Effie Elizabeth Lindsey Austin Jim Seaberry Frieda Austin Darrell Austin Harrell Austin Charles Austin 78 Wayne Leon Austin 101 Robert Carn Woosley 83 Callie Bee McElw ain Woosley Lynn Rickard Sally Rickard

My Family Pedigree

Grundy Guy Orther Woosley Gladys Woosley Aubrey Woosley Walt Rickard Charles Rickard Frank Rickard Lee Rickard Joe Rickard Rickard Carrico 76 Martha Leona Woosley Austin 87 RD Rickard 80 Jean Evon Thomas Coffman 63 Marjorie Loraine Thomas Rickard 72 Billy John Thomas 72 Charlie Louis Thomas 76 Grace Louise Thomas Totton Andrea Lynn Jones Austin Michael Shane Austin 50 Bryan Keith Austin Kimberly Sutherlin Austin 54 Roger Wayne Austin 57 Janet Louise Rickard Austin 54 Susan Kay Franklin Rickard Young Dale Young Patty Rickard David Rickard Tonya 8 6 Stephen Michael Austin Luke Andrew Austin Benjamin Thomas Austin Jessica Marie Hale 25 Bryan Adam Austin 23 Ashley Adam Mischelle Hardin Austin 20 Alyssa Kendall Austin Michael Moore 4 Hadley Mischelle Hardin Austin Michael Moore 22 Kelly Jan Austin 20 Laura Beth Austin Jackie Kaye Knight Young Underw ood Brandon Shane Young Christina Michelle Rickard David Andrew Rickard 12 8 Ely Brandon Gavin Lee Young Young

Human Traits

• The phenotype of an organisms are only partly governed by the genotype. Many traits are strongly influenced by the environment, nutrition, and exercise.

– Environmental effects are not inherited though, only genes are inherited.

Human Genes

• The Human Genome- our complete set of genetic information- includes thousands of genes.

Human Blood Groups

• • The Rh blood group is determined by a single gene with two alleles- positive and negative.

– The positive Rh+ allele is dominant.

The ABO blood group has three alleles I A , I B , and i. – Alleles I A and I B are codominant. These alleles produce molecules known as antigens on the surface of red blood cells.

– The i allele is recessive, and produces no antigen.

Some Autosomal Disorders in Humans

Type of Disorder Disorders caused by Recessive alleles Disorder Albinism Cystic Fibrosis Major Symptoms Lack of pigment in skin, hair, and eyes.

Excess mucus in lungs, digestive tract, liver; increased susceptibility infections.

Phenylketonuria Accumulation of phenylalanine in tissues; lack of normal skin pigment; mental retardation dwarfism Disorders caused by Dominant alleles Achondroplasia Disorders caused by Codominant alleles Huntington’s Mental deterioration and uncontrollable movements; symptoms usually appears in middle age.

Sickle Cell Disease Mishapen, or sickled, red blood cells’ damage to many tissues

From Gene to Molecule

• In some diseases, such as cystic fibrosis, and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder.

Exit Slip

• If a woman with type O blood and a man with type AB blood have children, what are the children’s possible genotypes?

Warm Up

Need new warm up question

Human Genes and Chromosomes

• Sex-Linked Genes- genes located on the sex chromosomes are said to be sex-linked.

– Males have just one X chromosomes, thus all X-linked alleles are expressed in males, even if they are recessive.

Inheritance of Colorblindness

Human Genes and Chromosomes

• • Barr Bodies- an inactive form of an X chromosome in females.

Nondisjunction- when homologous chromosomes fail to separate in meiosis.

– Nondisjunction can lead to an abnormal number of chromosomes.

Exit Slip

• Distinguish between sex-linked disorders and sex chromosome disorders.