Mendelian Patterns of Inheritance

Gregor MendelGenetics is the

study of heredity.Mendel was an

Austrian monk who studied genetics in the 1860s using garden peas.◦Had no knowledge of

cells or chromosomes.◦Did not have a

microscope.

Mendel knew that…Each male

flower part produced pollen (sperm)

Each female flower part produced egg cells.

FertilizationFertilization: During sexual

reproduction, when sperm and egg cells join to produce a new cell.

Pea flowers are self-pollinating.◦The seeds produced from self-

pollination inherit all of their characteristics from the single “parent”.

Cross-PollinationMendel had true-

breeding plants, that when allowed to self-pollinate, would produce identical offspring to themselves

Cross Pollination◦Mendel was able to

produce seeds that had 2 different parents, each with different characteristics.

Genes and DominanceTrait = a specific characteristic

that varies from one individual to the next.◦Examples: Height, Eye Color, Skin

ColorMendel studied seven pea plant

traits, each with two contrasting characters.

He crossed plants with each of the seven contrasting characters and studied their offspring.

Theories of InheritanceTheories of inheritance in Mendel’s

time:◦Based on blending◦Parents of contrasting appearance produce

offspring of intermediate appearanceMendel’s findings were in contrast

with this◦He formulated the particulate theory of

inheritance◦Inheritance involves reshuffling of genes

from generation to generation

Mendel’s F1 Crosses on Pea Plants

Mendel’s F1 Crosses on Pea Plants

Mendel’s first Conclusion

Mendel's first conclusion was that biological inheritance is determined by factors that are passed from one generation to the next.Today, scientists call the factors that determine traits genes.

AllelesEach of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait.The different forms of a gene are called alleles.

Mendel’s Second ConclusionMendel’s second conclusion is called the principle of dominance.The principle of dominance states that some alleles are dominant and others are recessive.

Segregation

◦ Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation.

◦ The traits controlled by recessive alleles reappeared in one fourth of the F2 plants.

Copyright Pearson Prentice Hall

P GenerationF1 Generation

Tall Tall Tall Tall Tall TallShort Short

F2 Generation

Segregation

Segregation

• Each individual has a pair of factors (alleles) for each trait

• The factors (alleles) segregate (separate) during gamete (sperm & egg) formation

• Each gamete contains only one factor (allele) from each pair

• Fertilization gives the offspring two factors for each trait

Modern View of GeneticsEach trait in a pea plant is controlled

by two alleles (alternate forms of a gene)

Dominant allele (capital letter) masks the expression of the recessive allele (lower-case)

Alleles occur on a homologous pair of chromosomes at a particular gene locus

◦ Homozygous = identical alleles (AA or aa)

◦ Heterozygous = different alleles (Aa)

A capital letter represents the dominant allele for tall.A lowercase letter represents the recessive allele for short.In this example, T = tallt = short

Genotype vs. Phenotype Genotype

◦ Refers to the two alleles an individual has for a specific trait

◦ If identical, genotype is homozygous

◦ If different, genotype is heterozygous

Phenotype

◦ Refers to the physical appearance of the individual

Punnett SquaresTable listing possible genotypes

resulting from a cross.

◦All possible sperm genotypes are lined up on one side

◦All possible egg genotypes are lined up on the other side

◦Every possible zygote genotypes are placed within the squares

Monohybrid CrossOnly looks at one trait.Punnett square contains 4 boxes.

Individuals with recessive phenotype always have the homozygous recessive genotype

However, individuals with dominant phenotype:

◦ May be homozygous dominant, or

◦ Heterozygous

Monohybrid Test CrossUsed to determine the genotype

of an individual showing the dominant phenotype.

Cross the organism with the undetermined dominant genotype with an organism with the recessive genotype.

Example Test CrossSuppose in humans, tongue-

rolling is a dominant trait…we will represent it with a T.

We want to find the genotype of a man who can roll is tongue.◦ He could be TT…or Tt.

To test this, we would perform a cross between him and an individual who cannot roll their tongue. (tt)

Dihybrid CrossPredicts outcomes of a cross

looking at 2 traits at the same time.◦Example: Flower height and color,

Hair color and eye color.Punnett Square contains 16

boxes.

Law of Independent AssortmentGenes for different traits are

inherited independently of each other.

What does this mean?◦Things like eye color and hair color

are not “linked” or inherited together. Just because you inherit blue eyes from your mom does not mean you will also inherit her blonde hair.

Example

In pea plants, Round Seeds ( R) are dominant to Wrinkled Seeds (r) and Yellow Seeds (Y) are dominant to green seeds (y)

Practice ProblemIn humans, Brown hair (H) is

dominant to Blonde hair (h) and Brown eyes (B) are dominant to Blue eyes (b).

Create a punnett square crossing parents who are both heterozygous for both traits.

Exceptions to Mendel’s RulesIncomplete DominanceCo-DominanceMultiple AllelesPolygenic Traits

Incomplete DominanceWhen one allele is not

completely dominant over the other.

The heterozygous genotype shows a BLEND of the two homozygous phenotypes.

Example

CodominanceBoth alleles contribute to the phenotype.

◦ In certain varieties of cows, the allele for a brown coat is codominant with the allele for a white coat.

◦ Heterozygous cows look like:

Multiple AllelesMore than 2 possible alleles exist

for a single gene.An individual cannot have more

than 2, but there are more than 2 possible alleles that can combine together.◦I.E. Not just a dominant and a

recessive.

ExampleIn blood typing, there are 3

alleles:◦A (IA), B (IB), and O (i).

The combination of 2 alleles that each individual person has, determines their blood type.

Blood Type Genotype Can Receive Blood From

A IA IA

IA i

A or O

B IB IB IB i

B or O

AB IA IB A, B, AB, or O

O ii O

Polygenic TraitsTraits controlled by more than one gene.

◦Example: Skin Color.

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

AaBbCcAaBbCc

20/64

15/64

6/64

1/64

Fract

ion

of

pro

geny

Sex-linked TraitsNormally

carried on the X Chromosome.

Females = XXMales = XY