© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 2 Genetics Gregor Mendel is considered the father of genetics Published his work in 1866.

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 2

Genetics Gregor Mendel is

considered the father of genetics

Published his work in 1866

Seven Characters Mendel Studied

Mendel’s Laws

First Law: Segregation the two alleles of a trait separate from each other

during the formation of gametes, so that half of the gametes will carry one copy and half will carry the other copy

Mendel’s Second Law: Independent Assortment

genes located on different chromosomes are inherited independently of one another

Human Genetics human somatic cell: 46

chromosomes 22 pairs same in both

males/females: autosomes 1 pair different: sex

chromosomes: females XX, males XY

Aneuploidy: errors during meiosis, leads to individuals with abnormal # chromosomes; chromosome 21: Down

syndrome chromosome 22: individuals with

delayed development and mental impairment

XXX female, taller than average XXY Klinefelter syndrome, male,

sterile, with female characteristics and diminished mental capacity

XO Turner syndrome, female, sterile with diminished stature

XYY males, fertile and normal appearance

human heredity: study with family trees or pedigree: sex-linked/autosomal or dominant/recessive

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Essential Terms in Genetics

Genotype—the genetic makeup Phenotype—observable characteristics

Diploid: Two copies of each chromosome, such as somatic cells Homologous pair, Paternal & Maternal

Haploid: Reproductive cells have only one copy of each chromosome

Genetic Cross: When two individuals are mated

P generation—parent generation F1 generation—first generation F2 generation—second generation

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Alleles: Different versions of a given gene Human blood groups: A,B, AB,

and O Three alleles of one gene:

IA: A type sugar IB: B type sugar i: neither A or B

Homozygote—same two alleles (AA or aa)

Heterozygote—two different alleles (Aa)

dominant traits are capitalized, lower-case letter is for recessive trait: flower color in peas

P signifies purple, p signifies white

Gametes: 2 from each father/mother: P or p Punnett square: shows the possible gametes

and genotypes of potential offspring from a cross

Huntington’s disease: Huntington’s disease: progressive deterioration of brain cells, affected individuals only need 1 copy (dominant) Expression in the

30’s

Sickle-cell anemia: Sickle-cell anemia: affected individuals (homozygous recessive) with defective hemoglobin that produces a sickle-shape red blood cell which cannot move through the blood vessels and tends to clot Heterozygous individuals resistance to malaria infection

Hemophilia: Hemophilia: sex-linked (mostly on the X chromosome) trait, creates a difficulty of the body to stop any bleeding

Steps to answer properly a genetic problem:1. Assign letters to each allele, dominant and recessive,

with the corresponding phenotype2. Determine the phenotype and genotype of the parents3. Write down the possible gametes produced by the

parents, 4. write down your cross (mating), and split the letters of

the genotype for each parent and put them outside a p-square

5. Determine the genotype and phenotype of the offspring and estimate their probabilities

6. Enjoy your accomplishment

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Extensions of Mendel’s Laws Many alleles do not show complete dominance

Incomplete dominanceCodominancePleiotropyEpistasisEnvironmental effectsPolygenic traits


1. Polygenic Traits

• Trait is determined by two or more genes

• A continuous distribution– Skin color– Height


2. Pleiotropy

• A single gene influences a variety of traits:– Albinism— Absence of melanin

• Skin color• Vision problems

– characteristic of inherited disorders…..cystic fibrosis, sickle-cell anemia


3. Epistasis

• Gene interaction affects the phenotype• such that one gene contributes to or masks

the expression of the other gene • Alleles:

– BB or Bb = black– bb = brown– cc = white


4. Environmental Effects

• Internal and external conditions influence phenotype– Temperature– Chemicals– Nutrition

• arctic foxes: only produce fur pigment when temperatures are warm


5. Incomplete Dominance

• Heterozygote is an intermediate

• Horses• Snapdragons


6. Codominance

• Both alleles are expressed

• Seen in blood types– IAIA or IAi = type A– IBIB or IBi = type B– ii = type O– IAIB = type AB

– gene ABO blood type in humans: affects the membrane of red blood cells which act as recognition markers for cells in the immune system

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PRACTICE #1

In peas the allele for purple flower (P) is dominant to the allele for white flower (p).

What gametes will be produced by a plant that has white flowers?

PRACTICE #2

What kinds of gametes can this genotype produce? AaBB

PRACTICE #3

Suppose that flower color is inherited by simple dominance and that purple flowers are dominant to white flowers.

If a homozygous recessive individual is crossed with a homozygous dominant individual,

what is the probability of obtaining a purple flowered offspring?

Show all 5 steps for full credit.

PRACTICE #4

Pea plants are tall if they have the genotype TT or Tt, and the are short id they have genotype tt.

A tall plant is mated with a short plant.

Half the offspring are tall and half short.

What do you know about the tall plant?

Show all 5 steps for full credit

PRACTICE #5

Assume tall (T) is dominant to dwarf (t).

If a homozygous dominant individual is crossed with a dwarf,

What will the offspring look like?


PRACTICE #6

Flower color in snapdragons is an example of incomplete dominance.

If a pink-flowered plant is crossed with a pink-flowered plant,

What will the progeny of plants look like?


PRACTICE #7

Color in snapdragons is inherited by incomplete dominance.

Suppose you cross a red (R)flowered snapdragon with a white flowered snapdragon (r).

ALL of the offspring have pink flowers. What can you conclude about the genotypes

of the parents and offspring? Show all 5 steps for full credit.

PRACTICE #8

Wanda has type A blood, and her husband has type B blood.

Is it possible for this couple to have a child with type O blood?

Explain your answer, and show all 5 steps for full credit.

PRACTICE #9

A man who has type B blood and a woman who has type A blood have a child with type O blood,

How is this possible?

Explain your answer, and show all 5 steps for full credit.

PRACTICE #10

Hemophilia is a sex linked trait.

If a woman is a carrier of hemophilia and she marries a man who does not have hemophilia,

what is the probability that her sons inherit the disease?


Practice questions1. Gregor Mendel studied the garden pea plants because:A) pea plants are small, easy to grow, grow quickly, and produce lots of flowers and

seeds. B) he knew about studies with the garden pea that had been done for hundreds of

years, and wanted to continue them, using math - counting and recording differences.

C) he knew that there were many varieties available with distinctive characteristics. D) all of the above.

2. Human height shows a continuous variation from the very short to the very tall. Height is most likely controlled by:

A) epistatic genes. B) environmental factors. C) sex-linked genes. D) multiple genes.

3. In the human ABO blood grouping, the four basic blood types are type A, type B, type AB, and type O. The blood proteins A and B are _________________

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4. Nondisjunction:A) occurs when homologous chromosomes or sister

chromatids fail to separate during meiosis. B) may lead to Down syndrome. C) results in aneuploidy. D) All of the above.

5. A red carnation and a white carnationproduce offspring that are all pink. The typeof inheritance pattern occurring is:

A. Complete dominanceB. Incomplete dominanceC. Codominance

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6. If an allele for tall plants (T) is dominant to short plants (t), what offspring would youexpect from a TT x Tt cross?A. ½ tall; ½ shortB. ¾ tall; ¼ shortC. All tall

7. Fur color in rabbits shows incomplete dominance.FBFB individuals are brown, FBFW individuals arecream, FWFW individuals are white. What is theexpected ratio of a FBFW x FWFW cross?A. 3 white : 1 brownB. 3 white : 1 creamC. 2 white : 2 cream

© 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e 2 Genetics Gregor Mendel is considered the father of genetics Published his work in 1866.

Documents

cross slide

dominantrecessive slide

white slide

bleeding slide

males xy slide

normal appearance slide

malaria infection slide

chromosomes chromosome