11

Introduction to Genetics (Chap 11)

The Study of Heredity

Do any two zebrashave the samestripe pattern?

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A Quick Review

A. Do you look like your parents ? Why? Because they gave you your DNA.

B. How is this information transferred? Through sex

C. What do we call a segment of DNA that codes for a protein?

GeneD. Where are the genes located?

On the chromosomesE. How many chromosomes do humans have?

46 F. How many genes are on each chromosome?

Depends but it is estimated about 30,000 genes in a human genome.

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I. Introductory TermsA. Genetics is the study of

heredityB. Gene - a segment of DNA

that codes for a protein.C. Allele - different forms or

physical expressions of a gene.

1. Example: Your thumb has 2 alleles: bent and straight.

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I. Introductory Terms cont.D. Trait - a specific

characteristic that varies from one individual to another

E. Hybrid - The offspring of crosses between parents with different traits.

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A. “Father of Genetics”.1. Austrian monk

studied math & science.

2. Studied variation of traits in pea plants.

3. Showed that the inheritance of traits follows certain laws.

II. Gregor Mendel

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The Work of Gregor Mendel

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B. Mendel’s Experiment1. Cross-bred pea plants with different

traits.2. Studied 7 plant traits (seed/pea

shape, color, height, etc.) of the hybrid offspring

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth GreenAxial Tall

Mendel’s Seven F1 Crosses on Pea Plants

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2. The 1st generation (F1) had the characteristics of only one of the parent plants (P)

Why did the “short” trait appear in the F2 generation but not in the F1 generation?

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

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3. The 2nd generation (F2) had characteristics of BOTH parent plants (P). Always 3:1 ratio.

Why did the “short” trait appear in the F2 generation but not in the F1 generation?

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

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4. Segregation (to separate).a. During gamete (sex cell) formation,

alleles separate. Each gamete carries only a single copy of each gene.

What are the two types of

Gametes (sex cells)?

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4. Segregation (to separate)b. Each F1 plant produces two types of

gametes—those with the allele for tallness and those with the allele for shortness.

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III. Mendel’s Conclusions.A. Principal of Unit characters -

Every trait such as height, color, etc. are inherited separately

* Not necessarily true!

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B. Principal of Dominance1. Some unit characters (alleles)

mask others when they are present these are called dominant alleles.a. Represented by capitol letters of

the dominant trait.b. The recessive allele is

represented by the same letter but in lower case.

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B. Principal of Dominance2. In garden peas tall is dominant

over short (TT, Tt, tt).

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C. Principal of Segregation- In the formation of sex cells (gametes), one member of each allele separates into different sex cells.

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D. Principal of Independent Assortment- Genes for different traits can separate independently during the formation of gametes.

* Also, not necessarily true!

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Quiz! Gametes are also known as

A. genes.B. sex cells. C. alleles.D. hybrids.

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Quiz! The offspring of crosses

between parents with different traits are called A. alleles.B. hybrids.C. gametes.D. dominant.

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Quiz! In a cross of a true-breeding tall

pea plant with a true-breeding short pea plant, the F1 generation consists ofA. all short plants.B. all tall plants.C. half tall plants and half short

plants.D. all plants of intermediate

height.

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IV. Probability & Punnett Square

How do geneticists use the principles of probability?The principles of probability can be

used to predict the outcomes of genetic crosses.

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How do geneticists use Punnett squares?Punnett squares can be used to

predict and compare the genetic variations that will result from a cross.

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Punnett Squares

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A. Punnett Squares1. Punnett squares can be used to

predict and compare the genetic variations that will result from a cross.

F1 Parents

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2. A capital letter = dominant allele (T=tall).

3. A lowercase letter = recessive allele (t=short).

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4. Gametes from each parent are on the top and left side.

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5. Possible gene combinations (genotypes) are in the four boxes.

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B. Genetic Terms1. Genotype - genetic makeup

(represented by letters) describes the two alleles of a gene.

What are the genotypes?

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2. Phenotype – What it looks like (physical traits).

What are the phenotypes?

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3. Homozygous – both alleles are the same (purebred).a. Homozygous dominate – genotype

has two dominate alleles.b. Homozygous recessing – genotype

has two recessive alleles.

What is the homozygous dominant genotype?

What is the homozygous recessive genotype?

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4. Heterozygous – genotype (hybrid) means one dominant and one recessive allele.

What is the heterozygous genotype?

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The plants have different genotypes (TT and Tt), but they have the same phenotype (tall).

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1. 1/4 of the F2 plants have two alleles for tallness (TT).

2. 1/2 have one allele for tall (T), and one for short (t).

3. 1/4 of the F2 have two alleles for short (tt).

C. Probability & Segregation

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Probability can be used to predict a. average outcome of many

events.b. precise outcome of any event.c. how many offspring a cross will

produce.d. which organisms will mate with

each other.

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Compared to 4 flips of a coin, 400 flips of the coin isa. more likely to produce about 50%

heads and 50% tails.b. less likely to produce about 50%

heads and 50% tails.c. guaranteed to produce exactly 50%

heads and 50% tails.d. equally likely to produce about 50%

heads and 50% tails.

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Organisms that have two different alleles for a particular trait are said to be a. hybrid.b. heterozygous.c. homozygous.d. recessive.

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Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall? a. 100%b. 50%c. 0%d. 25%

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The Punnett square allows you to predict

a. only the phenotypes of the offspring from a cross.

b. only the genotypes of the offspring from a cross.

c. both the genotypes and the phenotypes from a cross.

d. neither the genotypes nor the phenotypes from a cross.

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V. Other Mendelian Topics

A. Independent Assortment1. Two-Factor Cross: True-breeding pea

plants with two different genes were bred: round yellow peas (RRYY) X wrinkled green peas (rryy).

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2. All F1 offspring were heterozygous for round yellow seeds.

Fig 11-9

Round-Yellow

Wrinkled-green

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3. Heterozygous F1 plants (RrYy) were crossed to see if alleles would separate independently in the F2 generation.

3a The ratio 9:3:3:1is what is expectedfor traits inherited independently

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In Mendel’s experiment, the F2 generation produced the following:• some seeds that were round and yellow• some seeds that were wrinkled and

green• some seeds that were round and green• some seeds that were wrinkled and

yellow

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4. Mendel’s conclusion:a. Principal of Independent

Assortment- - Genes for different traits separate independently (in gametes) & create genetic differences in plants & animals.

b. *This rule is broken if the two genes being crossed are close to each other on the same chromosome

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In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F2 generation is evidence for:A. the two traits being inherited

together.B. an outcome that depends on the

sex of the parent plants.C. the two traits being inherited

independently of each other.D. multiple genes being responsible

for each trait.

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1. Incomplete Dominance – When one allele is not completely dominant over another.

a. The heterozygous phenotype is between the two homozygous phenotypes.

B. Beyond Dominant & Recessive Alleles

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A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).

Incomplete Dominance

WW

RR

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2. Codominance – Both alleles are equally dominant and expressed separately at 50:50 ratio.

Co-dominance Rhododendron

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3. Multiple alleles - Genes that are controlled by more than two alleles

a. In some populations more that two alleles can exist that control a trait (e.g. coat color in rabbits).

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Different combinations of alleles result in the colors shown here.

Full color: CC, Ccch, Cch, or CcChinchilla: cchch, cchcch, or cchcHimalayan: chc, or chchAIbino: cc

KEY

C = full color; dominant to all other alleles

cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles

ch = Himalayan; color in certain parts of the body; dominant to c allele

c = albino; no color; recessive to all other alleles

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4. Polygenic Trait - Traits controlled by two or more genes

a. Skin color in humans is a polygenic trait controlled by more than four different genes.

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11–3 Traits controlled by two or

more genes are called 1. multiple-allele traits.2. polygenic traits.3. codominant traits.4. hybrid traits.

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In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have 1. pink flowers.2. white flowers.3. half white flowers and half red

flowers.4. red flowers.

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A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of1. incomplete dominance.2. multiple alleles.3. codominance.4. a polygenic trait.

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Mendel's principles apply to 1. pea plants only.2. fruit flies only.3. all organisms.4. only plants and animals.

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MEIOSIS

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Remember Mitosis? Cell Division (2N)

Growth (child adult) Repair (wounds/cuts) Replace (old cells i.e. red blood

cells) Parent cell is the SAME as

daughter cells (2N). ALL body tissues EXCEPT gametes

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I. MeiosisA. Cell division (N).

1. Produces ONLY gametes (egg & sperm)

2. Parent cell (2N) is NOT the same as resulting gametes

3. Only occur in ovaries & testes

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B. Meiosis is a reduction division process.

1. The number of chromosomes in a diploid (2N) cell are reduced by half in a haploid (N) cell (humans: 4623).

2. Meiosis has one DNA (chromosomes) duplication and two division cycles.

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Meiosis Overview

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C. Meiosis I1. Prophase 1

a) Chromatin coils to form chromosomes. There are 2 copies of each chromosomes from each parent (homologous pairs).

b) Each chromosome pairs with its matching homologous chromosome to form a tetrad (four chromatids).

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C. Stages of meiosis1. Prophase 1

c) When tetrads form in meiosis I, they exchange portions of their chromatids in a process called crossing over.

d) Crossing-over produces new combinations of alleles.

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Crossing Over Drawing

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Crossing Over

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2. Metaphase 1a) Chromosomes line up in the

middle of the cell (homologous pairs).

3. Anaphase 1a) Homologous pairs pull apart

toward the poles.

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4. Telophase 1/Cytokinesis 1a) Nucleus reforms and splits into

two haploid cells (N).5. Meiosis I results in 2 haploid (N)

daughter cells, each with half the number of chromosomes as the original cell.

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D. Meiosis II – There is NO interphase. DNA does NOT duplicate!!!

1. Prophase 2 – Chromatin coils to form chromosomes pairs.

2. Metaphase 2 – chromosomes line up at the cell center.

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3. Anaphase 2 – Sister chromatids pull apart.

4. Telophase 2/Cytokinesis 2a) Four haploid cells (N) with half the

number of chromosomes are formed into gametes.

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Meiosis Movie

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II. Ovum (Egg) formation “Oogenesis”

A. In female gametes the process is the same except at:1. Telophase 1 - one cell gets most of

the cytoplasm.a) Results in secondary egg and

one polar body.2. Telophase 2 - same thing occurs

B. This leaves three non-functioning polar body and one large ovum.

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Spermatogenisis

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Review of Oogenesis

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III.Mitosis/Meiosis Cycle

Adult(2N)

Eggs(N)

Sperm(N)

Zygote(2N)

Meiosis

Mitosis

Fertilization

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V. Human GeneticsA.Recessive Disorders1.Sickle cell anemia – In hemoglobin,

valine is substituted for glutamic acid.a.What is the resulting phenotype?

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2. Tay sachs (1:3600 central European Jews)a. A defective enzyme prevents

normal formation of the brain causing mental retardation, blindness and early death.

Why are these genetic disorders primarily found in only certain populations (blacks & Jews) ?

Why are most disorders due to recessive alleles?

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Trisomy

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B. Huntington's Disease Caused by a dominant allele. If everyone with the allele dies, how

does the disease persist?If one of your parents have the disease what

are the odds of you getting it?

C. Sex linked disorders These genes are found on the X chromosome.1. Color blindness2. Hemophilia

H hhh

Parents Genotypes Hh and hh Hh

Hhhhhh

50%

Chance

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D. Multiple Alleles (eg. Blood Type)

Blood Type Genotype Surface Protein Antibodies**

A

B

AB

O

AA or Ao

BB or BoAB

oo

A

B

A & B

--

b

a

--a & b**The antibodies are only produced when the

recipient is exposed to the wrong surface proteins.Who is the universal

donor?Who is the universal recipient?

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E. Chromosomal Disorders Most common are non-disjunction Non-disjunction - the failure of

chromosomes to separate during meiosis. Daughter cells have an abnormal number of chromosomes

1. Turner’s Syndromea) Females missing one of the X

chromosomes (0 X)b) Sterile and very short but have

normal intelligence.

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2. Klinefelter’s Syndromea) Males with an extra X

chromosome (X X Y).b) Sterile, can have several

female characteristics, like breasts, little body hair and a female shaped body.

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Man with Klinefelter’s Syndrome

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3. Down Syndromea. an extra twenty first chromosome b. common in children of older mothers.c. dwarfs, mentally retarded, and usually

have heart problems.

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VI. Genetic ScreeningA. Amniocentesis

1. Involves removing some fluid from the woman’s womb and then conducting genetic tests on it.

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B. Karyotyping1. Get a small blood sample *see

amniocentisis.2. Separate the white blood cells and

induce cell division.3. Kill, stain and place them on a slide.4. Examine and photograph cells in

metaphase.5. Blow up picture and cut out

chromosomes.6. Arrange them in order of size.