Chapter 14~Mendel & The Gene Idea

The Origins of Genetics

Heredity: the passing of traits from parents to offspring

Gregor Mendel did experiments with garden peas 100+ years ago in a monastery in Vienna.

1st to develop rules that accurately predict patterns of heredity.

Mendel Animation

Mendelian genetics (characters and traits)

Gene- DNA segment that codes for particular protein production (heritable feature, i.e., fur color)

Allele- variant for a gene (blue vs. brown eyes)

P generation (parents) F generation (filial

generation: offspring)

• Crossing pea plants

1

5

4

3

2

Removed stamensfrom purple flower

Transferred sperm-bearing pollen from stamens of white flower to egg-bearing carpel of purple flower Parental

generation(P)

Pollinated carpelmatured into pod

Carpel(female)

Stamens(male)

Planted seedsfrom pod

Examinedoffspring:all purpleflowers

Firstgenerationoffspring(F1)

APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color.

TECHNIQUETECHNIQUE

When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers.

TECHNIQUERESULTS

Mendel’s Work with Pea Plants

1st experiments were monohybrid crosses: involve one trait like flower color or seed shape

Used true breeding parents: plants that always showed one form of the trait

When cross-pollinating two true breeding parent plants (one purple & one white)

F1: the offspring from the cross, only showed the purple flower form of the trait

F1 generation was allowed to self-pollinate and the F2 generation had the traits in a ratio of 3:1 purple:white

P Generation

(true-breeding parents) Purple

flowersWhiteflowers

F1 Generation (hybrids)

All plants hadpurple flowers

F2 Generation

Mendel discovered

Mendel observed the same pattern– In many other

pea plant characters

Mendel’s Theory

1. For each inherited trait, an individual has two copies, one from each parent

2. There are alternative versions of genes, alleles

3. When two different alleles occur together, one trait may be expressed while the other is not. Dominant: trait expressed if one or two alleles were present. Recessive: only expressed if two alleles are present.

First, alternative versions of genes

Account for variations in inherited characters, which are now called alleles

Allele for purple flowers

Locus for flower-color geneHomologouspair ofchromosomes

Allele for white flowers

4. Alleles for each trait separate independently when forming gametes. Gametes only carry one allele for each trait (haploid).

Homozygous: when an individual has two of the same alleles for a trait.

Heterozygous: when an individual has two different alleles for a trait

Genotype: the set of alleles that an individual has for a gene (e.g. PP, Pp, pp)

Phenotype: the physical appearance of a trait (purple vs. white flowers)

The phenotype is determined by the genotype– PP = purple– Pp = purple– pp = white

Phenotype versus genotype

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1

Leading to the Law of Segregation

The alleles for each character segregate (separate) during gamete production (meiosis).

Mendel’s Law of Segregation

Punnett square: predicts the results of a genetic cross between individuals of known genotype

P Generation

F1 Generation

F2 Generation

P p

P p

P p

P

p

PpPP

ppPp

Appearance:Genetic makeup:

Purple flowersPP

White flowerspp

Purple flowersPp

Appearance:Genetic makeup:

Gametes:

Gametes:

F1 sperm

F1 eggs

1/21/2

Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.

Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.

3 : 1

Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation.

This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm.

The testcross

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

If PP,then all offspring

purple:

If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:

p p

P

P

Pp Pp

PpPp

pp pp

PpPpP

p

p p

APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.

RESULTS

Testcross: breeding of a recessive homozygote to a dominate phenotype (but unknown genotype)

The Law of Independent Assortment

Law of Segregation involves 1 character. What about 2 characters?

Monohybrid cross vs. dihybrid cross

The two pairs of alleles segregate independently of each other.

Genetic Variation

Independent assortment: the homologous chromosomes distribute randomly to the gametes

Crossing over and independent assortment increase the number of genetic combinations

Genetic variation is important for evolution– Alleles (different versions of a genes) lead to

different traits being expressed.

The Multiplication and Addition Rules Applied to Monohybrid Crosses

The multiplication rule– States that the probability that two or more

independent events will occur together is the product of their individual probabilities

Probability in a monohybrid cross

Can be determined using this rule

Rr

Segregation ofalleles into eggs

Rr

Segregation ofalleles into sperm

R r

rR

RR

R1⁄2

1⁄2 1⁄2

1⁄41⁄4

1⁄4 1⁄4

1⁄2 rr R r

r

Sperm

Eggs

The rule of addition

States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

Solving Complex Genetics Problems with the Rules of Probability

We can apply the rules of probability– To predict the outcome of crosses involving

multiple characters

A dihybrid or other multicharacter cross– Is equivalent to two or more independent

monohybrid crosses occurring simultaneously

In calculating the chances for various genotypes from such crosses– Each character first is considered separately

and then the individual probabilities are multiplied together

Inheritance patterns are often more complex than predicted by simple Mendelian genetics– The relationship between genotype and

phenotype is rarely simple

Extending Mendelian Genetics for a Single Gene

The inheritance of characters by a single gene– Complete dominance

• Occurs when the phenotypes of the heterozygote and dominant homozygote are identical

May deviate from simple Mendelian patterns

Heredity Can Be Complicated

Codominance: both traits are displayed Multiple alleles: genes with three or

more alleles. Example; blood types. Incomplete dominance: when both

alleles have an influence over the phenotype

The human blood group ABO

Codominance– Two dominant

alleles affect the phenotype in separate, distinguishable ways

Multiple alleles: more than 2 possible alleles for a gene (Ex: human blood types)

Single gene genetics (examples)

Incomplete dominance: appearance between the phenotypes of the 2 parents. (Ex: snapdragons)

Multiple gene genetics

• Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia

Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color

Multiple gene genetics

Polygenic traits: several genes influence one trait. Examples: eye color, height, weight, hair and skin color

Nature and Nurture: The Environmental Impact on Phenotype

Another departure from simple Mendelian genetics arises– When the phenotype for a character depends

on environment as well as on genotype

The norm of reaction– Is the phenotypic range of a particular

genotype that is influenced by the environment

Studying Heredity• Pedigree: a family

history that shows a trait over several generations

• Autosomal traits appear in both sexes equally (on autosomes)

• Sex-linked trait: a trait whose allele is located on a sex chromosome.

Pedigree Analysis

• A pedigree– Is a family tree that describes the

interrelationships of parents and children across generations

Inheritance patterns of particular traits

• Can be traced and described using pedigrees

Ww ww ww Ww

wwWwWwwwwwWw

WWor

Ww

ww

First generation(grandparents)

Second generation(parents plus aunts

and uncles)

Thirdgeneration

(two sisters)

Ff Ff ff Ff

ffFfFfffffFF or Ff

ff FForFf

Widow’s peak No Widow’s peak Attached earlobe Free earlobe

(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

Recessively Inherited Disorders

• Many genetic disorders– Are inherited in a recessive manner

• Recessively inherited disorders– Show up only in individuals homozygous for

the allele

• Carriers– Are heterozygous individuals who carry the

recessive allele but are phenotypically normal

Cystic Fibrosis

• Symptoms of cystic fibrosis include– Mucus buildup in the some internal organs– Abnormal absorption of nutrients in the small

intestine

Sickle-Cell Disease

• Sickle-cell disease– Affects one out of 400 African-Americans– Is caused by the substitution of a single amino

acid in the hemoglobin protein in red blood cells

• Symptoms include– Physical weakness, pain, organ damage, and

even paralysis

Dominantly Inherited Disorders

• One example is achondroplasia– A form of dwarfism that

is lethal when homozygous for the dominant allele

Huntington’s disease

• Is a degenerative disease of the nervous system

• Has no obvious phenotypic effects until about 35 to 40 years of age

Multifactorial Disorders

• Many human diseases– Have both genetic and environment

components

• Examples include– Heart disease and cancer

• Pedigrees– Can also be used to make predictions about future

offspring (genetic counseling)• Other tests also available

– Amniocentesis– Chorionic villi sampling (CVS)

(a) Amniocentesis

Amniotic fluidwithdrawn

Fetus

Placenta Uterus Cervix

Centrifugation

A sample ofamniotic fluid canbe taken starting atthe 14th to 16thweek of pregnancy.

(b) Chorionic villus sampling (CVS)

FluidFetalcells

Biochemical tests can bePerformed immediately onthe amniotic fluid or lateron the cultured cells.

Fetal cells must be culturedfor several weeks to obtainsufficient numbers forkaryotyping.

Severalweeks

Biochemicaltests

Severalhours

Fetalcells

Placenta Chorionic viIIi

A sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.

Suction tubeInserted throughcervix

Fetus

Karyotyping and biochemicaltests can be performed onthe fetal cells immediately,providing results within a dayor so.

Karyotyping