Mendelian Genetics

Blending Hypothesis

Used in early 1800’s to explain how offspring inherit traits from parents.

A yellow flower plant crossed with a red flower plant will produce plants that have orange flowers

Orange flower plants will produce orange flower plants

But there were many exceptions like a red flower plant that produced yellow flowered offspring

Gregor Mendel

Gregor Mendel was an Austrian monk who is often called the "father of genetics" for his study of the inheritance of traits in pea plants.

• Between 1856 and 1863 Mendel cultivated and tested some 28,000 pea plants.

• He was the first person to predict how traits are transferred from one generation to the next.

• Applied an experimental approach to studying genetics.

• He studied only one trait at a time to control variables, analyzed data mathematically. Statistics.

Why Pea plants?

Pea Plants have a short life cycle.

Pea plants self fertilized their own flowers.

They are easy to see and recognize the traits.

Mendel’s 7 pea plant traits

Flower color

Flower position

Seed color

Seed shape

Pod shape

Pod color

Stem length

Trait – a specific inherited characteristic that can vary in individuals

True breeding Pea plants

Because pea plants are self pollinating they inherit all their characteristics from their single parent. Mendel noticed that tall plants always produced seeds that grew to be tall and that short plants always produced seeds that produced short plants. All yellow seed plants produced plants with yellow seeds. In other words all his plants were true-breeding

Mendel’s experiment

Mendel cut off the male flower reproductive parts to prevent self pollination.

He then dusted pollen from another plant onto the flower – cross pollination.

Produced seeds that had 2 different parents.

• http://www.cactus-art.biz/note-book/Dictionary/aaa_Dictionary_pictures/cross_pollination.jpg

Each of the seven pea plant traits had two contrasting characters.

Flower color purple or white

Flower position axial or terminal

Seed color yellow or green

Seed shape round or wrinkled

Pod shape inflated or constricted

Pod color yellow or green

Stem length tall or short (dwarf)

Mendel crossed plants with the contrasting traits.

Monohybrid crosses – study one trait at a time

When Mendel crossed a purple and a white flower he always got a purple flower for the offspring

Mendel called each of the original pairs of plants the parental (P) generation.

The offspring from these crosses were known as the F1 generation. (F for filial, Latin for son or daughter)

Mendel noticed that all the offspring only had the character of one of the parents. The character of the other parent seemed to disappear.

Mendel’s 1st PrincipleThe law of unit factors - each organism

has two factors for each of its traits

• We now know these factors are genes located on the chromosomes.

• These different gene forms are called alleles.

• Alleles are located in different copies of a chromosome; one from ♂ parent, one from ♀ parent

Gene is the chemical factor that determines traits. Flower color gene

Alleles are the different forms of a gene.

purple flower allele or white flower allele

Mendel’s 2nd Principle

The law of dominance - only one factor was visible in the F1 generation, the dominant trait

The dominant trait is the visible or observable trait.

The recessive trait is the hidden trait, masked by the dominant trait.

Mendel allowed the F1 generation to self pollinate and planted the resulting seeds

When the F2 plants grew he observed that about three-fourths of the F2 plants had the dominant trait and one-fourth had the recessive trait

Mendel’s 3rd Principle

Mendel concluded that the paired factors separate during the formation of gametes and that when 2 gametes combine during fertilization the offspring have two factors for each trait. This is called the Law of Segregation– a pair of factors is separated or segregated during the formation of gametes

Mendel crossed plants that differed in two traits such as flower color and seed color. His data showed that the dominant traits do not have to appear together. A yellow seed (dominant) could appear in a plant with wrinkled seeds (recessive). Mendel concluded that factors for individual characteristics are not connected.

Mendel’s 4th Principle

Factors for different traits separate independently of one another during the formation of gametes. This is called the Law of independent assortment.

Mendel and Modern Genetics

Most of what Mendel concluded agrees with what we now know about molecular genetics. Molecular genetics is the study and function of chromosomes and genes.

Mendel’s Law of independent assortment is supported by the independent assortment of chromosomes to gametes during meiosis.

Genotype

An organism’s genetic makeup is its genotype. The genotype consists of alleles that the organism inherits from both parents.

Letters are used to represent alleles. Capital letters refer to dominant alleles and lowercase letters refer to recessive alleles.

The actual letter used to selected to represent an allele is typically the first letter of the dominant trait. Like P for purple flower and p for white flower.

Genotype vs. Phenotype

The genotype for a plant with purple flowers maybe PP or Pp because the P allele is dominant.

The genotype for a plant with white flowers an only be pp.

An organism’s appearance is its phenotype. The phenotype of a PP or a Pp plant is purple flowers.

The phenotype of a plant with pp is white flowers.

Homozygous Vs Heterozygous

When both alleles of a pair are alike the organism is homozygous.

PP homozygous dominant

pp homozygous recessive

When both alleles of a pair are different the organism is heterozygous

like Pp

Probability

Probability is the likelihood of that a specific event will occur.

number of times an event is expected to happenProbability

number of times an event could happen

Probability can be expressed a a percentage, a decimal or a fraction eg. 50%, 0.50 or 1/2

Probability

In Mendel’s experiment the dominant trait of yellow seed color appeared in the F2generation 6,022 times and the green seed color appeared 2,001 times. The total number of individuals was 8,023 (6,022 + 2,001).

The probability that the dominant yellow seed color will appear is 6,022 3

0.75 or 75% or 8,023 4

Punnett Squares

The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square. Punnett squares can be used to predict and compare the genetic variations that will result from a cross.

The types of gametes produced by each parent are shown along the top (for males) and along the left side (for females). The possible gene combinations for the offspring, appear in the four boxes that make up the square

♂ father

T  T 

♀ mother t   Tt  Tt

t  Tt  Tt 

Homozygous dominant vs Homozygous recessive

TT (tall) plant vs tt (short) plant = 100% heterozygous (Tt) offspring

Homozygous dominant vs Heterozygous TT (tall) plant vs Tt (tall) plant = 50% heterozygous (Tt) offspring & 50% homozygous dominant (TT)

♂ father

T  T 

♀ mother TT  TT  TT

t  Tt  Tt 

Heterozgous vs HeterozygousTt (tall) plant vs Tt (tall) plant = 25% TT (tall), 50%

Tt (tall), and 25% tt (short) 3 tall: 1 short phenotypic ratio

♂ father

T  t

♀ mother T  TT  Tt

t  Tt  tt 

Test cross

How can you determine whether on organism with a dominant trait is homozygous dominant or heterozygous? Perform a test cross with a homozygous recessive organism.

TestcrossIf the organism is homozygous dominant then a

test cross with a homozygous recessive will produce all heterozygous offspring, 100% tall

♂ father

T  T 

♀ mother t   Tt  Tt

t  Tt  Tt 

TestcrossIf the organism is heterozygous then a test cross with a homozygous recessive will produce 50% heterozygous dominant and 50% homozygous recessive heterozygous offspring, 50% tall 50%

short phenotypes♂ father

T  t

♀ mother t   Tt  tt

t  Tt  tt 

Dihybrid crosses

Crossing organisms with 2 different traits

The principle of independent assortment states that during gamete formation any allele for one trait can be paired with either allele for another trait

Punnett squares with 16 blocks can be used to determine the probability of the offspring.

Dihybrid Crossheterozygous round yellow seed (RrYy) plant with a another heterozygous round yellow seed (RrYy)

plant

♂ father RrYy

 RY  Ry  rY  ry

 RY  RRYY  RRYy  RrYY  RrYy

♀ mother  Ry  RRYy  RRyy  RrYy  Rryy

 rY  RrYY  RrYy rrYY   rrYy

 ry  RrYy RRyy   rrYy  rryy

Intermediate Inheritanceincomplete dominance

Seen in many flower species

four o’clocks

impatients

snapdragons

Also seen in Andalusian chickens

Intermediate Inheritance

Neither allele is dominant

Andalusian chickens have 3 phenotypes Black, White, & “Blues”

A “C” for color is used because neither the black or white allele is dominant

CB is the black allele

CW is the white allele

♂ father CBCB

CB CB

♀ mother

CWCW CW  CBCW   CBCW

CW  CBCW CBCW

All F1 offspring have “blue” phenotype and are heterozygous CBCW

♂ father CBCW

CB CW

♀ mother

CBCW CB  CBCB   CBCW

CW  CBCW CWCW

50% of F2 offspring are heterozygous and have the “blue” phenotype25% are homozygous black 25% are homozygous white

CRCR (red) x CWCW (white) CRCW (pink)

Multiple Alleles

For many genes several alleles exist. Like blood type. A carbohydrate chain seen on the membrane of Red Blood cells.

IA-allele carbohydrate A on red blood cells.

IB-allele carbohydrate B on red blood cells

i-neither allele recessive to both IA & IB

Both IA & IB are dominant and are referred to as codominant because both traits are expressed in individuals with bothe A and B alleles

Blood Types

Type A blood can have the possible genotypes IAIA homozygous or IAi heterozygous.

Type B blood IBIB or IBi

Type AB blood IAIB

Type O blood ii

♂ father IBIB

IB IB

♀ mother

IAIA IA  IAIB   IAIB

IA IAIB IAIB

Type B

Type A

♂ father IBi

IB

♀ mother

IAi IA  IAIB   IAi

i IBi ii

Polygenetic InheritanceMany traits are affected by multiple genesLike height, skin color, intelligence, and

personalityExample suppose genes A, B & C which are

dominant control dark skin pigmentation. Genes a, b & c are recessive and control light pigmentation. There would be 64 possible combinations. AABBCC has the maximum amount of melanin and very dark skin. aabbcc has the lowest amount of melanin and very light skin. A person with AaBbCc would have an intermediate skin color

Gametes  ABC   ABc   AbC   Abc   aBC   aBc   abC   abc 

ABC 6 5 5 4 5 4 4 3

ABc 5 4 4 3 4 3 3 2

AbC 5 4 4 3 4 3 3 2

Abc 4 3 3 2 3 2 2 1

aBC 5 4 4 3 4 3 3 2

aBc 4 3 3 2 3 2 2 1

abC 4 3 3 2 3 2 2 1

abc 3 2 2 1 2 1 1 0

Poylgenetic Inheritance

When there is a lot of variation in a trait like skin color, or height than it is usually polygenetic Inheritance and multiple genes are responsible for the trait.

Genes and the EnvironmentAn individuals genotype can depend on the

environment and their genes.Nutrition, exercise, sunlight, illness can

result in changes in phenotype.Siamese cats and Himalayan rabbits coat

color is affected by temperature. Colder regions of their bodies have darker coat color.

Bothe genotype and environmental factors can play a role in phenotype

Meiosis and Mendel

Chromosome theory of Inheritance states that genes are on chromosomes and the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns.

Chromosomes undergo segregation and independent assortment during meiosis and support Mendel’s two principles of the same name.

Crossing over

During meiosis and prophase I homologous chromosomes pair up and genetic information can be exchanged between chromatids. The alleles for a gene for a particular trait reside at the same location called the gene loci. If an individual has the same alleles on both homologous chromosomes at the gene loci then they are considered homozygous and heterozygous if the alleles at a gene loci are different.

Genetic linkage and crossing over

The only way alleles on the same chromosome could assort themselves independently is if crossing over occurs in meiosis and separates them.

The tendency for alleles on one chromosome to be inherited together is called genetic linkage. The closer two genes are on a chromosome the greater the genetic linkage. The farther apart the genes are on the chromosome the more likely that a crossover event will separate them.

Sex-Linked Traits

Genes located on sex chromosomes are sex-linked genes.

Thomas Hunt Morgan studied fruit flies. Most fruit flies have red eyes, white eyes are very rare.

He crossed a white eyed male with a red eyed female all offspring were red eyed red eyes were dominant.

When Morgan bred the F1 offspring he got a 3:1 ratio for phenotype red eyes: white eyes but all the white eyes were always male.

Sex linked disorders

HemophiliaRe-Green Color blindnessDuchenne’s Muscular DystrophyAll are more common in males because they

are carried on the X chromosome and not on the Y chromosome. Because males only get one X chromosome if they get they recessive gene they don’t have another X chromosome.

Hemophilianormal father carrier mother

♂ father XHy normal

XH Y♀ mother

XHXh XH  XHXH   XHY

carrier Xh XHXh XhY