Heredity

Biology 30

Early Theories of Inheritance

Aristotle (384-322 B.C.E.) proposed the first widely accepted theory of inheritance

• called pangenesis

• egg and sperm consist of particles called pangenes that come from all parts of the body.

• upon fertilization the pangenes develop into the parts of the body from which they are derived.

• egg and sperm consist of particles called pangenes that come from all parts of the body.

• upon fertilization the pangenes develop into the parts of the body from which they are derived.

Antony van Leeuwenhoek (1632-1723) discovered sperm in semen.

• he believed he thought he saw a complete miniature person called a homunculus inside the head of the sperm.

• other people of Antony’s time thought that the egg contained the entire person.

Mendelian Genetics

Gregor Mendel (1822-1884) a Augustinian monk in Brunn (Czech

Republic), Austria his research laid the foundation for

modern genetics and the science of inheritance.

for seven years he bred pea plants (Pisum sativum) and analyzed the results.

Mendelian Genetics

Mendel focuses on seven different traits of pea plants..

Mendelian Genetics

Mendel let plants self-pollinate to ensure they were true breeding.

• true breeding plants exhibit the same characteristics generation after generation.

• Mendel called • true breeding plants the parental or P generation

• the first offspring first filial or F1 generation

• If the F1 generation were to pollinate the offspring would be called the second filial or F2 generation

Mendelian Genetics

• Mendel called the first offspring first filial or F1 generation

• If the F1 generation were to pollinate the offspring would be called the second filial or F2 generation

• because all of Mendel’s initial crosses only involved one trait we call them monohybrid crosses.

• Mendel observed that:

• for every trait crossed the F1 generation only showed one of the two parental traits.

ie. if plants with round seeds were crossed with plants of wrinkled seeds the F1 generation would only have plants of round seeds.

Mendelian Genetics

• Mendel observed that:

• for every trait crossed the F1 generation only showed one of the two parental traits.

ie. if plants with round seeds were crossed with plants of wrinkled seeds the F1 generation would only have plants of round seeds.

• even though the F1 generation had a copy of both genes only one was expressed.

• Mendel called this characteristic dominant.

allele: one of alternative forms of a gene.

the gene for wrinkled and the gene for round peas are alleles.

Mendelian Genetics

• even though the F1 generation had a copy of both genes only one was expressed.

• Mendel called this characteristic dominant.

allele: one of alternative forms of a gene.

the gene for wrinkled and the gene for round peas are alleles.

dominant trait: a characteristic that is expressed when one or both alleles in an individual are the dominant form

~ dominant alleles are indicated by an uppercase letter (R)

Mendelian Genetics

dominant trait: a characteristic that is expressed when one or both alleles in an individual are the dominant form

~ dominant alleles are indicated by an uppercase letter (R)

• Mendel called the characteristic that was not expressed recessive

recessive trait: a characteristic that is expressed only when both alleles in an individual are the recessive form.

• Mendel concluded that one form showed complete dominance.

• an individual with one dominant and one recessive (Rr) had the same characteristics as one with two dominant forms (RR)

Mendelian Genetics• Mendel concluded that one

form showed complete dominance.

• an individual with one dominant and one recessive (Rr) had the same characteristics as one with two dominant forms (RR)

Mendel’s Traits

Trait Dominant Recessive

Stem Length Tall (T) Short (t)

Pod Shape Inflated (I) pinched (i)

Seed Colour Yellow (Y) Green (y)

Flower Position

Axial (A) Terminal (a)

Flower Colour Purple (P) White (p)

Seed Shape Round (R) Wrinkled (r)

Pod Colour Green (G) Yellow (g)

Mendelian Genetics

Important Definitions

Homozygous: having identical alleles for the same gene

Heterozygous: having different alleles for the same gene.

Genotype: the genetic complement of an organism

Phenotype: the observable characteristics of an organism

Segregation: the separation of alleles during meiosis.

Mendelian GeneticsGenotype: the genetic complement of an organism

Phenotype: the observable characteristics of an organism

Segregation: the separation of alleles during meiosis.

Law of Segregation Mendel’s First Law

• All individuals have two copies of each factor (gene). These copies segregate (separate) randomly during gamete formation, and each gamete receives one copy of every gene.

in 1909 Danish Botanist Wilhem Ludwig Johannsen called Mendel’s “factors” genes

Mendelian Genetics

Analyzing Genetic Crosses

Reginald Punnett (1875-1967)

• devised a visual way to analyze the results of crosses, called a Punnett’s square.

Mendelian Genetics

P Generation♀

♂

Phenotypic Ratio

Genotypic Ratio

Punnett Squares

•are used to predict the genotype and phenotype of potential off-spring

•very useful when producing economically important cattle and plants.

Mendelian GeneticsTrait Dominant

Phenotype

Genotype(s) Recessive

Phenotype

Genotype(s)

Stem Length Tall TT (homozygous)

Short tt (homozygous)Tt (heterozygous)

Pod Shape InflatedII (Homozygous)

Pinched ii (homozygous)Ii (hetorozygous)

Seed Shape RoundRR (Homozygous)

Wrinkled rr (homozygous)Rr (Heterozygous)

Flower Colour PurplePP (Homozygous)

White pp (homozygous)Pp (herozygous)

In order to see recessive phenotypes the genotype must be homozygous

Test Cross a test cross of an individual of unknown genotype to an

individual that is fully recessive the phenotypes of the F1 generation of the test cross

reveals whether the unknown genotype is homozygous or heterozygous

example:

• you have a white ram (white is dominant “W” and black is recessive “w”) and want to know if it is heterozygous or homozygous for breeding purposes.

example:

• you have a white ram (white is dominant “W” and black is recessive “w”) and want to know if it is heterozygous or homozygous for breeding purposes.

do a test cross by crossing your unknown ram with one showing a recessive phenotype.

• it must have a recessive genotype (ww)

Test 1♀ ww

♂

WwWw Ww

ww ww

Test 2♀ ww

♂

WWWw Ww

Ww Ww

If the ram is Heterozygous it will produce:

Phenotypic ratio: 50% white 50% black, or 2:2 or 1:1

Genotypic ratio:

2:2 or 1:1 hetero:homo recessive

If the ram is Homozygous it will produce:

Phenotypic ratio: 100 % White

Genotypic ratio:

100% heterozygous

Test 1♀ Rr

♂

rrRr rr

Rr rr

Analyze:

Heterozygous Seed shape crossed with a recessive seed shape

Analyze:

What are the predicted phenotypes and genotypes?

Phenotypic Ratio

•50% round 50% wrinkled or 2:2 or 1:1, ½, 2/4,

Genotypic Ratio

•50% hetero: 50% homo recessive, 2:2 or 1:1, hetero 2/4 or ½ homo recessive 2/4 or ½

Example Problem A horticulturist has seeds from

a cross but does not know the genotype of the phenotype of the parents. Use the following information to figure out the parental phenotype and genotype

Offspring Phenotype

Numbers

round-seed peas

5472

wrinkle-seed peas

1850

Solution because there is two different phenotypes one the

parents must not be homozygous dominant

Offspring Phenotype

Numbers

round-seed peas

5472

wrinkle-seed peas

1850

Solution 5472/1850 = 2.96

2.96/1 or 2.96 : 1

~ 3 : 1

to get a 3:1 ratio both parents must be heterozygous

Tester♀ ??

♂

RR

R? R?

R? R?

Solution because there are two

different phenotypes one the parents must not be homozygous dominant

Mendelian Genetics

Proof♀ Rr

♂

RrRR Rr

Rr rr

Proof♀ rr

♂

RrRr Rr

rr rr

3:1 Phenotypic ratio 1:1 (50%) Phenotypic ratio

Mendel’s Second Law

• The Law of Independent Assortment Mendel also crossed plants of two traits.

• because two traits are involved in these crosses they are called a dihybrid cross.

Mendel crossed true breeding tall plants that had green pods (TTGG) with true breeding short plants that had yellow pods (ttgg) to produce the F1 generation

in this case the true breeding plants will produce only one type of gametes

TTGG → will produce gametes with the TG genes

ttgg → will produce gametes with the tg genes

♀

♂tg tg

TG TtGg TtGg

TG TtGg TtGg

the phenotypic ratio of the F1 generation:

100% tall and green pods the genotypic ratio of the F1

generation

100% heterozygous

Mendel then crossed the F1 generation to produce an F2 generation

in this case the plants of the F1 generation produce four different types of gametes

TtGg → will produce gametes with the:TG genes (tall, green)Tg genes (tall, yellow)tG genes (short, green)tg genes (short, yellow)

TtGg → will produce gametes with the:

TG genesTg genes

tG genes

tg genes

♀

♂ TG Tg tG tg

TG TTGG TTGg TtGG TtGg

Tg TTGg TTgg TtGg Ttgg

tG TtGG TtGg ttGG ttGg

tg TtGg Ttgg ttGg ttgg

♀

♂ TG Tg tG tg

TG TTGG TTGg TtGG TtGg

Tg TTGg TTgg TtGg Ttgg

tG TtGG TtGg ttGG ttGg

tg TtGg Ttgg ttGg ttgg

Phenotypes Tally

Tall & Green Pods

9

Tall & Yellow Pods

3

Short & Green Pods

3

Short & Yellow Pods

1

TT = tall GG = green

Tt = tall Gg = green

tt = short gg = yellow

for every dihybrid cross that Mendel carried he got the 9:3:3:1 ratio (when he crossed the F1 generation).

• this ratio is what is expected if the segregation of alleles for one gene had no influence on the segregation of alleles of another gene.

Law of Independent Assortment

• The two alleles of one gene segregate (assort) independently of the alleles for other genes during gamete formation

Law of Independent Assortment

• The two alleles of one gene segregate (assort) independently of the alleles for other genes during gamete formation

Pleiotropic Genes• a gene that affects more than one characteristic

• example: Sickle-cell anemia• the normal hemoglobin is produced by the allele HbA

• in sicke-cell anemia the individual has two copies of the mutated allele Hbs

Pleiotropic Genes

• a gene that affects more than one characteristic

• example: Sickle-cell anemia

• the normal hemoglobin is produced by the allele HbA

• in sicke-cell anemia the individual has two copies of the mutated allele Hbs

• the mutation cause abnormally shaped hemoglobin that cannot deliver oxygen to the cells.

• causes fatigue, enlarged spleen, pneumonia and major organ damage.

• a heterozygous individual has resistance to malaria but an increased chance of having homozygous recessive offspring.