Basic Principles of Heredity• Gregor Mendel (1822 – 1884) –

Austrian monk who first discovered the basic rules of inheritance – his work was rediscovered in 1900 and he came to be known as the Father of Genetics

Definitions of Genetics Terms• locus – specific location of a gene on a

chromosome homologous chromosomes carry the same

type of gene, located at the same place (same type, but may not be identical)

• alleles – alternative forms of the same gene ex – human blood types produced by three different alleles A, B, and O

• homozygous – an organism that has the same allele on both homologous chromosomes at a given gene locus

• heterozygous – when the two homologous chromosomes have different alleles at a given gene locus – also called a hybrid

•pure-breeding (true-breeding) – during Mendel’s time, commercial sources sold homozygous plants – Mendel started with these types of plants

•phenotype – the physical appearance of an organism

•genotype – the actual combination of alleles carried by an organism

•genome – the whole of the genetic information of an organism

Mendel’s Experiments• worked with pea

plants – advantages:

• easy to grow

• many contrasting traits to study

• easy to control pollination

• Mendel chose to study seven clearly contrasting inherited traits and subjected the results to mathematical analysis

• Mendel began by crossing true-breeding plants with purple flowers to true-breeding plants with white flowers

Purple x White P generation (parental)

• F1 generation (first filial) all had purple flowers

• Mendel said that the purple trait was dominant – it covered up the white trait

• he said the white trait (the one hidden) was recessive

• Mendel then crossed the F1 generation

• Purple x Purple F1

• F2 generation (second filial) were a mixture of purple and white

Inheritance of Dominant and Recessive Alleles – How can we explain Mendel’s

results?• Each trait is determined

by pairs of genes. Each individual has two genes for each trait, one on each homologous chromosome (one from mom, one from dad).

• Mendel’s Law (Principle) of Segregation – pairs of genes on homologous chromosomes separate from each other during gamete formation.

• Each gamete receives only one of each parent’s pair of genes for each trait. When a sperm fertilizes an egg, the offspring receives one allele from the father one from the mother

• When two alternative forms of a gene are inherited, one (dominant) may mask the expression of the other (recessive) but it does not change the recessive allele. The unchanged recessive allele may be passed to offspring in the individual’s gametes

Monohybrid Crosses - inheritance of two

alleles of a single locus

•We use alphabet letters to represent alleles. Capital letters for dominant traits and lower case for recessive traits. Always use the same letter for the same trait.

ex - PP x pp P generation

P – purplep - white

F1 are all Pp

•Punnett square – used to figure out the possible combinations of eggs and sperm at fertilization

•a cross between a homozygote and a heterozygote always results in a 1:1 genotypic ratio

for example:P PP x PpF1 1 PP: 1 Pp

P pp x PpF1 1 pp: 1 Pp

• phenotype does not always reveal the genotype

• ex – heterozygotes – must do a test cross to determine genotype

• test cross – an individual of unknown genotype showing a dominant phenotype is crossed with an individual who is homozygous recessive

Dihybrid Crosses • Mendel also analyzed crosses involving

alleles of two or more loci

• Dihybrid Crosses – a mating between individuals with different alleles at two lociround R yellow Ywrinkled r green y

•Mendel then crossed the heterozygous F1

•Mendel’s Law of Independent Assortment – each allele pair segregates independently during meiosis when the traits are located on separate chromosomes

XRXr

XYXy

XR Xr

XY Xy

RY ry

XRXr

XyXY

XR Xr

Xy XY

Ry rY

F1 RrYy x RrYy

F1 RrYy x RrYy

Possible gametes:

RYRyrYry

RY Ry rY ry

RY

Ry

rY

ry

RrYy x RrYy1 RRYY

2 RRYy

2RrYY

4RrYy

1RRyy

2Rryy

1rrYY

2rrYy

1rryy

9/16 Round and Yellow

3/16 Round and Green

3/16 Wrinkled and Yellow

1/16 Wrinkled and Green

When crossing two heterozygotes in a dihybrid cross showing independent assortment (traits located on separate chromosomes), you always get a:

9:3:3:1 phenotypic ratio

When crossing heterozygote with a homozygote in a dihybrid cross showing independent assortment, you always get a:

1:1:1:1 phenotypic ratio

Using the Rules of Probability to Solve Genetics Problems

• Product Rule – if two or more events are independent of each other, the probability of their both occurring together is the product of their individual probabilities Events are independent if the occurrence of one

does not affect the probability that the other will occur

• example: How many of the 16 combinations in Mendel’s cross will produce the wrinkled, yellow phenotype (crossing 2 heterozygotes)?

•wrinkled is recessive and expected in ¼ of the offspring in a monohybrid cross

• yellow is dominant and expected in ¾ of the offspring

• so… multiply ¼ x ¾ = 3/16

•3 out of 16 will be wrinkled and yellow

•Example: Trihybrid cross (assume R/r – red/white, Y/y – yellow/green, C/c – round/wrinkled)

•cross 2 heterozygotes RrYyCc x RrYyCc

•What fraction of offspring will have red flowers, yellow seeds, and wrinkled seeds?

•probability for red flowers ¾

•probability for yellow seeds ¾

•probability for wrinkled seeds ¼

•¾ x ¾ x ¼ = 9/64 9 out of the 64 possible combos

Linked genes • genes that occur on the same chromosome do

not assort independently during meiosis – they tend to be inherited together (linked)

• genes in a particular chromosome tend to be inherited together and constitute a linkage group

• sometimes linked genes are NOT inherited together – this can be explained by crossing-over – results in appearance of new combinations of genes that were previously linked

• crossing-over results in genetic recombination – the generation of new combinations of alleles by the exchange of DNA between homologous chromosomes

• individuals with new genetic combinations are called recombinants

•Ex: cross between individuals with two linked genes

TtBb x ttbb

Frequency of crossing over can be used to map chromosomes

• Crossing over results in recombinants• Recombination frequency = # of recombinants

total # of offspring• Recombination frequencies reflect the distances between genes on

a chromosome• The farther apart two genes are, the higher the probability that a

crossover occur between them resulting in higher recombination frequency

• Recombination frequencies can be used to determine distances between genes on a chromosome to create a chromosome map

Sex Determination • most animals have a special pair of sex

chromosomes (all other chromosomes are autosomes)

• members of one sex are homogametic – have a pair of similar sex chromosomes and produce only one type of gamete

• females of many species (including humans) have two X chromosomes – form only “X” gametes

• genotype is XX

• members of the other sex are heterogametic – have two different sex chromosomes

• males of many species (including humans) have one X chromosome and one Y chromosome – form ½ “X” gametes and ½ “Y” gametes

• all individuals require at least one X chromosome and the Y is the male-determining chromosome

• X and Y chromosomes are not truly homologous

• different in shape, size, and genetic constitution

• Male determines the sex of the baby

• ½ the sperm are Y and ½ are X

• all of the eggs are X

• if an X sperm fertilizes the egg, the baby is XX and is a girl

• if a Y sperm fertilizes the egg, the baby is XY and is a boy

Sex-linked traits • genes that are on one sex chromosome but not on

the other (also called X-linked)• Y chromosome carries few genes other than those

that determine maleness• X chromosome bears many genes that have

nothing to do with being female (has genes for color vision and blood clotting)

• females receive two alleles for traits found on the X chromosome (one from mom and one from dad)• she can be either homozygous or

heterozygous• males only receive one allele for traits on the X

chromosome (from mom) because the other chromosome is Y (which came from dad) and does not carry the X traits

• males express all of their X chromosome alleles whether or not they are dominant (because there is only one X chromosome)

• Sex-linked traits are usually expressed in the male

• Example: Color vision is inherited on the X chromosome

• if a female inherits the recessive gene for colorblindness, she still has a second gene to possibly make up for it

Xc XC

• a female must inherit two recessive genes (one from mom and one from dad) in order to express a sex-linked trait

Xc Xc

• males only have one X chromosome (comes from mom) – if the allele on that chromosome is recessive, it will be expressed (he will be colorblind)

Xc Y

Gene Interactions – many traits are not just controlled by one pair of genes that are dominant or recessive – many traits are controlled by many pairs of genes cooperating to control the expression of a single trait

1. Incomplete dominance – alleles are not always completely dominant or recessive

crossing red snapdragons with white snapdragons produces all pink offspring

when the heterozygote has a phenotype that is intermediate between those of the two parents, the genes show incomplete dominance

2. Codominance – the heterozygote simultaneously expresses

the phenotypes of both homozygous parents (ex. roan coat color in horses and cows, human blood types)

3. Multiple alleles – when three or more alleles exist for a given gene within a population (ex. eye color in fruit flies, human blood types)

•any one individual has only up to two alleles

Human Blood Types

• involves 3 alleles – A, B, O

• produces 4 blood types: A, B, AB, O

• blood types are based on antigens (surface proteins) on the red blood cells

•exposure to an antigen causes an immune response – the body produces antibodies to destroy the antigen

•immune system of each person is insensitive to the surface proteins (antigens) of its own cellsblood type antigens plasma antibodies

A A anti-B B B anti-A AB A and B none O none anti-A and B

•if a person receives the wrong blood type in a transfusion, the antibodies react with the antigen and cause agglutination (clumping) of cells

1.type A can receive O, A2.type B can receive O, B3.type AB can receive all types (universal

recipient)4.type O can receive only O (universal donor)

Inheritance of blood types • 3 alleles of the same gene: A (IA),

B (IB), and O (i)

• IA and IB are codominant

• IA and IB are both dominant over i phenotype genotypetype O iitype A IAIA or IAitype B IBIB or IBitype AB IAIB

Rh Factors•at least 8 different kinds of Rh

antigens located on surface of RBCs – the most important is antigen D (85% of US residents of Western European descent are Rh-positive)

•Rh negative (Rh-) – do not have antigen D will produce anti-D antibodies if

exposed to Rh+ blood

•Rh positive (Rh+) – have antigen

•Rh incompatibility may result if an Rh- woman is pregnant with an Rh+ child

Other Gene Interactions1. Polygenic inheritance – occurs when multiple

independent pairs of genes have similar and additive effects on a single phenotype

examples include height, body form, skin color, eye color

2.Pleiotropy – occurs when single genes have multiple phenotypic effectsexample – cystic fibrosis and sickle cell anemia – one pair of alleles cause multiple symptoms

3.Epistasis – a gene at one locus prevents the phenotypic expression of a gene at a second locus example – coat color in mice Black (B) coat color is dominant to brown

(b) coat color the expression of either color is

dependent on another pair of alleles which controls the production of the pigment – C is dominant and causes deposition of pigment, c is recessive and results in no pigment being produced

therefore, if a mouse is cc, regardless if it is BB or Bb at the other locus, it will be an albino (will have no color at all; white with pink eyes)

Epistasis in Mice