Introduction to Genetics Chapter 11. Review Mitosis Meiosis

Introduction to Genetics

Chapter 11

Review

• Mitosis

• Meiosis

Mitosis

• Simple cell division, asexual reproduction

• # Chromosomes same as original cell

• DNA exactly same as original cell - clone

Meiosis

• Sexual reproduction

• # of chromosomes halved (haploid)

• One cell becomes 4

• DNA may not be same in all 4 because of crossing over

Sexual Reproduction

• Half of DNA (chromosomes) in the offspring from mother, half from father

Background

• Fertilization– Male and female haploid cells join during

sexual reproduction, become full cell with full set of DNA/chromosomes (human 46)

– Becomes a new cell– Cell divides(mitosis), division continues, cell

differentiation takes place

Cell differentiation

Cell differentiation

• http://www.pbslearningmedia.org/resource/tdc02.sci.life.stru.different/cell-differentiation/

???

• “Initially, all human fetuses are female.”

• T/F?

FALSE

• At conception, the egg is fertilized with a single sperm.

• At that point, the single cell as a full set of 46 chromosomes

• The baby that will be born is either XX or XY, so it is already determined

TRUE?

• The default pathway is to develop into a female.

• During the eighth week of gestation, the presence of a Y chromosome and a functional locus for the SRY gene product, also called the testes determining factor (TDF), determines if testicular development will occur.

Terms

• Trait specific characteristic of an organism – flower color, etc

• Gene section of DNA, part of a chromosome, that determines the trait

• Allele The different possible forms a trait can have. One allele from one of the homologous chromosomes (from mother), the other allele on the other homologous chromosome (from father)

Gene

Principle of Dominance

• Some alleles are dominant

• Some alleles are recessive

• Organism with one dominant allele will express that dominant trait

Segregation

• During meiosis, the matched alleles for a particular trait split, one goes to one haploid cell, the other allele goes to another haploid cell.

• During sexual reproduction, the next generation gets one allele for each trait from mother, and one allele for each trait fom father

The Principle of Independent Assortment

• Genes for different traits sort independently during formation of gametes

• Ex. – your brown hair gene is not attached to your blue eye gene. They are independent traits

• This accounts for genetic variations we see in organisms on Earth

Independent Assortment

• In addition to Crossing-Over, the process of Meiosis ensures that chromosomes are randomly assorted. The following images show three separate possibility for a single cell that has undergone meiosis. Look at all the different combinations.

Terms

• Homozygous – homo – same– Both of the alleles for a trait are the same

• YY = yellow yy = white• Caps are the dominant, lower case are recessive

• Heterozygous – hetero – different– One allele is dominant, one is recessive

• Yy = yellow

Math Review

• How do you represent a fraction?

• How do you represent a ratio?

• How do you represent a percentage?

Probability Review

• Coin– Multiplicative effect ½ x ½ x ½ = 1/8

• Die

• Dice

Terms

• Genotype– Genetic expression, or code for a trait – AA,

Tt, etc

• Phenotype– Physical expression, of the trait – tall, short,

blue, brown, etc

Gregor Mendel 1822-1884

• “Father of genetics”

• Developed what became

Laws of Inheritance

• Did extensive work with

pea plants, then bees

• His work was rejected

when he was alive

• Something like a pea plant is relatively simple genetically

• One gene, two allele types (dominant and recessive) defines that trait

• G = green, g = yellow.

Complications

• It is always not so simple, as we will see later.

Punnett Square

• Used to predict possible outcomes of a cross of two parents (P generation)

• Monohybrid Cross– Simple dominant/recessive– One gene, two alleles x 2 parents– 2x2 square– Split alleles, 1 parent across top, the other parent

down the side– Recombine possibilities inside grid– Calculate probability

Punnett Square Basics

• Genes are shown as letters• Capital is dominant• Lower case is recessive• P = parents, parental generation.• F = filial, pertaining to sons/daughters

• F1 then is the first generation of offspring

• F2 would then be a next “grandchild” generation

Beyond Basic Mendelian Genetics

• Incomplete Dominance– In a case of Aa, “in-between”– Red and white results in pink

• Codominance– 2 dominant genes result in a different trait– R(red), W(white) RW is brown

Complications

• Multiple Alleles– One gene site, all individuals have just two

alleles, but the population has multiple alleles for that gene see page 273

• Polygenic Traits– Multiple genes interact for the full trait

Human Eye Color

Eye Color

• The allele for brown eyes was once considered dominant

over the allele for blue eyes. The genetic basis for eye color is actually far more complex. At the present, three gene pairs controlling human eye color are known. Two of the gene pairs occur on chromosome pair 15 and one occurs on chromosome pair 19. The bey 2 gene, on chromosome 15, has a brown and a blue allele.

A second gene, located on chromosome 19 (the gey gene) has a blue and a green allele. A third gene, bey 1, located on chromosome 15, is a central brown eye color gene.

• Geneticists have designed a model using the bey 2 and gey gene pairs that explains the inheritance of blue, green and brown eyes. In this model the bey 2 gene has a brown and a blue allele. The brown allele is always dominant over the blue allele so even if a person is heterozygous (one brown and one blue allele) for the bey 2 gene on chromosome 15 the brown allele will be expressed. The gey gene also has two alleles, one green and one blue. The green allele is dominant to the blue allele on either chromosome but is recessive to the brown allele on chromosome 15. This means that there is a dominance order among the two gene pairs. If a person has a brown allele on chromosome 15 and all other alleles are blue or green the person will have brown eyes. If there is a green allele on chromosome 19 and the rest of the alleles are blue, eye color will be green. Blue eyes will occur only if all four alleles are for blue

eyes. This model explains the inheritance of blue, brown and green eyes but cannot account for gray, hazel or multiple shades of brown, blue, green and gray eyes. It cannot explain how two blue-eyed parents can produce a brown-eyed child or how eye color can change over time. This suggests

that there are other genes, yet to be discovered, that determine eye color or that modify the expression of the known eye color genes.

Blood type – Rh factor

• Specific clotting factor in blood

• Rh – one gene two alleles– Positive (dominant) and negative (recessive)– What % of the population do you think is

Rh+? (from Punnett square/monohybrid cross)

• The real number is 84%

ABO Blood Group

• IA, IB, i

• Multiple alleles, dominant/recessive and codominance all at one gene

• IA, IB are codominant

• i is recessive

Blood Types

• IA IA or IA i TYPE A

• IB IB or IB i TYPE B

• IA IB TYPE AB

• i i TYPE O

• O 46%

• A 40%

• B 10%

• AB 4%

ABO + Rh Blood Types

• O+ 38%• O- 7%• A+ 34%• A- 6%• B+ 9%• B- 2%• AB+ 3%• AB- 1%

Blood Types

• If two different blood types are mixed, the blood cells may begin to clump together in the blood vessels, a potentially fatal situation.

• Type A blood can donate to type A or AB.

• Type B blood can donate to type B or AB.

• Type AB blood can donate AB only.

• Type O blood can donate to anyone.

• A person with type O blood is said to be a universal donor.

• A person with type AB blood is said to be a universal receiver.

Blood Type A

Blood Type B

Blood Type O

Real Example – Sheltie Coat ColorThe variation of coat colors seen in the Sheltie is produced by a number of genes

Nine named gene loci for dog coat colors are commonly encountered in the literature.

Agouti, Merle, and Spotting Genes are the most important for determining color

http://www.athro.com/evo/gen/sheltiecalc.html

http://www.athro.com/evo/gen/sheltiecalc.html

Check on netflix

• Dog breeding

Dihybrid Cross

• Monohybrid cross – mono – one– One trait being studied at a time

• Dihybrid cross – di – two– Two factors/traits studied at one time– Same general procedure to Punnett square– Just more complicated

Dihybrid

• Need to cover all of the possibilities of combinations of the two traits for each parent

• Requires a 4 x 4 grid instead of a 2 x 2

• Because of: ……………………

Dihybrid Cross

• Pea Plant

• R – round, r – wrinkled

• Y – yellow, y – green

• Cross two completely heterozygous parents– RrYy x RrYy

First Inside Outside Last

• AaBb

Autosomal Disorders

• Genetic disorders due to genes

• The gene/allele does something to cause a problem or keep something from happening correctly

• Some are due to a recessive gene, some due to a dominant gene, some codominant

• Pg 345

Sex Linkage

• A chromosome is made up of many genes. Genes on the X are X-linked genes, if on y, y-linked

• On fruit flies white eye color only occurs on males, so it is ____ - linked.

X Linkage - Human

• About 1100 x-linked genes in humans

• Most of these are not for female anatomical traits

• Some of these are disorders – hemophilia, fragile x syndrome, red-green color blindness, and male pattern baldness.

• Many are DNA sequences for human function/development not discussed

Y linkage

• Y chromosome is much smaller, only ~2% human DNA.

• 50-60 genes mostly for male anatomy and fertility.