The Work of Gregor Mendel & Applying Mendel’s Principles

Unit 5C Genetic Inheritance

Heredity and Genetics• Heredity- the delivery of

characteristics from parent to offspring

• Genetics- the scientific study of heredity

• The Modern study of genetics was founded by a monk named Gregor Mendel in the mid- 1800’s

• Mendel studied genetics in pea plants

Mendel Changed Biology Forever by Working in the Monastery Garden!!

• Worked with garden peas- a “Model System”– Convenient to study– Easy to grow– Produce many offspring

quickly– Info from peas can tell

us about other organisms

Some other model systems are fruit flies, bacteria, mice, monkeys, etc.

A Little Bit About Peas• Pea plants have

flowers• Pea flower

Anatomy:– Anther- creates

pollen which contains sperm Ovary- Contains eggs

– Stigma- receives pollen to bring to the egg cells

Fertilization in Peas

• Normally the pollen of the pea plant will fertilize the eggs of the SAME pea plant (self pollination)– This will always produce offspring identical to the

parent plant- “true-breeding” plants• Mendel decided to “cross” pea plants– To cross means to cause one organism to reproduce

with another– Mendel used a paint brush to take pollen from one

plant and place it on the stigma of a different pea plant

Crossing Pea Plants- Cross Pollination

Setting Up A Cross• P Generation (parental)–

the original pair of plants• F1 Generation (“filial”

meaning son or daughter)- the offspring of the P generation

• F2 Generation- the offspring of a self-pollinated F1.

Traits

• Specific characteristics of an individual– Example: in pea plants- tall or short,

round or wrinkled– In humans, brown eyes, blue eyes

The Traits Mendel Studied

• Mendel crossed parents with opposite traits to study the following 7 traits in the offspring:

A- flower on side

T- flower at the end

• The offspring produced by crossing parents with different traits are called hybrids

Setting Up A Cross• P Generation (parental)– the original pair of

plants• F1 Generation (“filial” meaning son or

daughter)- the offspring of the P generation

Mendel’s F1 Results• All F1 offspring had the trait of only one parent• The trait of the other parent seemed to have

disappeared

Two Conclusions From These Data

1. An individual’s characteristics are determined by factors that are passed from one parental generation to the next. - Today, these “factors” are called genes- Each Trait Mendel studied was controlled by a

single gene- Each gene has two contrasting varieties- The different forms of the gene are called alleles- Example: the gene for height comes in alleles for

tall and short.

Two Conclusions From These Data (cont.)

2. The Principle of Dominance- some alleles are dominant and some are recessive- Organisms carry two alleles for each trait- Only one dominant allele needs to be present for that trait to be expressed (it “dominates” over the recessive allele)

- Two copies of the recessive allele must be present for that trait to be expressed.

TT Tt ttTwo dominant alleles

Two recessive alleles

One dominant and one recessive allele

Segregation• After seeing the trait of only

one parent show up in the F1, Mendel wanted to know if the other parent’s trait had disappeared.

• He allowed the F1 hybrid plants to self-pollinate.

• The offspring of the F1 cross are called the F2 generation (second filial)

Cross Pollination

Self Pollination

Segregation (cont.)

• The recessive traits reappeared in the F2 generation

• 25% of the F2 had the recessive trait

• He then asked, why did these recessive traits disappear in the F1 generation only to reappear in the F2 generation?

Cross Pollination

Self Pollination

Segregation• During gamete

formation, the pair of alleles for each trait will segregate, so that each gamete carries only 1 allele

Two Gametes Come Together During Fertilization

• When the gametes of the F1 come together in the F2, new combinations of alleles are created

• The recessive trait will show up when two recessive alleles come together

Two recessive alleles leads to the recessive trait

Allele Combinations

• Homozygous- organisms with two identical alleles for a particular gene (Ex: TT, tt)

• Heterozygous- organisms that have two different alleles for the same gene (Tt)

Genotype and Phenotype

• Genotype- the genetic makeup of an organism (Ex: TT)

• Phenotype- the observable characteristic or trait (Ex: Tall)

• A plant with the genotype _________ has a phenotype of short.

• A plant with the genotype of Tt has a phenotype of ____________.

Using Punnett Squares

• Probability can be used to predict the outcome of genetic crosses.

• Example: – Parent 1 is TT– Parent 2 is tt T

t

t

T

Both possible gametes from Parent 1

Both possible gametes from Parent 2

Genotypes:• What percent of the offspring are homozygous

dominant?• What percent of the offspring are heterozygous?• What percent of the offspring are homozygous

recessive?Phenotypes:• What percent are tall?• What percent are short?

Test Cross

• Used to determine if an individual with the dominant trait is homozygous or heterozygous

• Example: A pea plant is tall. Is its genotype TT or Tt?– Cross the tall pea plant with a short pea plant (tt)

to see the phenotypes of the offspring. If all offspring have the dominant phenotype then the parent was TT

Independent Assortment

• Mendel wanted to know if the segregation of one pair of alleles affects the segregation of another pair– Ex: if a parent pea plant is round and yellow does its

off have to have both round and yellow also, or can they have round and green, or wrinkles and yellow?

• Experiments testing how two genes are passed down are called two-factor or dihybrid crosses.

Dihybrid Cross: creating the F1• Mendel crossed plants with round, yellow

peas (RRYY) with plants with wrinkled, green peas (rryy)

Parent genotype

All possible gamete combinations

All offspring are heterozygous for both traits

Dihybrid Cross: creating the F2• F1 plants reproduced with each other (or

self pollinate)

Do all of the F2 have the same combinations of genes as the parents?

Thus, genes that segregate independently do not influence each other’s inheritance

Predictable Phenotypic Ratios for Offspring of Heterozygous Parents

• Monohybrid Cross:– Tt X Tt– Offspring ratio of dominant phenotype to

recessive phenotype will be 3:1• Dihybrid Cross:– TtGg X TtGg– Offspring ratio of both dominant to one dominant,

one recessive to both recessive will be 9:3:3:1

Independent Assortment Defined:

• This principle states that genes for different traits can segregate independently during the formation of gametes.

• This helps to explain why organisms with the same parents are genetically varied!

Summary of Mendel’s Contributions:

• Inheritance is determined by genes which are passed from parents to offspring

• Some forms of a gene (alleles) are dominant, some are recessive

• Adult organisms have two copies of each gene (one from each parent)– These genes segregate during gamete formation

• Alleles for different genes segregate independently.

• Mendel’s Principles apply to all organisms, not just plants!

Beyond Dominant and Recessive

• Not all genes follow the principle of dominance

• There are 4 exceptions to the principle of dominance:– Incomplete dominance– Codominance– Multiple alleles– Polygenic traits

Incomplete Dominance• Occur when one allele is not

completely dominant over the other

• Example: four o’clock flowers– Red (RR) x White (WW) = Pink

(RW) – Neither red nor white is

dominant– Heterozygous phenotypes are

a blending of the two homozygous phenotypes

Codominance• The phenotypes of both alleles are

expressed• Example: In some chickens, black

feathers are codominant with white feathers– Heterozygous chickens will have both

black and white feathers – Black and white are NOT blended, they

appear separately

Multiple Alleles• A gene with more than two alleles has “multiple alleles” • Individuals only have two copies of each gene• But, many alleles for the gene can exist in a population• Example:

– There are multiple alleles (3) for human blood type: IA, IB, and i.

– Alleles IA and IB are codominant– Each person inherits 2 of these alleles,

one from mom and one from dad.

Two Different Patterns of Inheritance (besides simple dominance)

Multiple Alleles and Codominance

Example:

Sex-Linked Inheritance

Example:

Key

XX=

XY=

C=

c=

Polygenic Traits

• Traits that are produced by interactions between multiple genes

• Examples: At least three genes work together to make the reddish-brown pigment in the eyes of fruit flies

– The variety of skin color in humans is because multiple genes interact to produce skin color.

Sex-Linked Inheritance

• Sex-linked genes- genes located on the sex chromosomes

• Genes on the Y are only found in males and are passed from father to son

• Genes on the X are found in both sexes, but remember… males have just one X

The Consequence of only having one X Chromosome

• Recessive disorders related to genes on the X chromosome are more common in males since they only have one X.

• Example: color blindness– 3 genes work together to produce color vision and ALL are

located on the X chromosome– If any of these alleles is defective in males, they will

experience problems seeing colors– In females, if one of these genes is defective, they still may

have a good copy of the allele on their other X chromosome!

Two Different Patterns of Inheritance (besides simple dominance)

Multiple Alleles and Codominance

Example:

Sex-Linked Inheritance

Example:

Key

XX=

XY=

C=

c=

Human Pedigrees• A pedigree is a chart that shows patterns of

inheritance in a family.

How to read a pedigree

• Circles represent females• Squares represent males• Shaded shapes mean the individual expresses

the trait• Not shaded shapes mean the individual DOES

NOT express the trait• Vertical lines connect parents to offspring• Horizontal lines represent a marriage

How to read a pedigree

•This pedigree is for the dominant “white forelock” trait•The grandfather expresses the trait•What is the genotype of the circles individual?•What do you think the genotype of the grandfather must be?