Genetics

Gregor Mendel (1822-1884) Austrian Monk, “Father of Genetics”

Bred Garden Peas (Pisum sativum)

Developed a simple set of rules to accurately predict patterns of heredity which form the basics of genetics

Years later we found that traits are determined by genes encoded in DNA

Heredity History Heredity – transmission of traits from parents to

offspring… before DNA was discovered it was one of the great mysteries of science!

Modeled experiments after British farmer T.A. Knight who bred garden peas and concluded purple flowers show a stronger tendency to appear than white flowers

Mendel used a mathematical approach and counted the number of each kind of offspring

Why did Mendel choose peas? Many easily distinguishable characteristics

2 possible traits (forms) of each characteristic

Quantitative – he could count plants with or with out trait

P. sativum were small, easy to grow, mature quickly, and produce lots of offspring

Pea plants can self-pollinate Male (pollen) and Female (pistil) parts are enclosed in the same

flower and it can fertilize itself

Pea plants can cross-pollinate Transfer pollen from one plant to the pistil of another plant

Anatomy of a flowering plant

Self pollination vs. Cross Pollination

Mendel’s Experimental Design Parental Generation (P generation):

ensure that ea/plant was true breeding – all offspring display only one form of the characteristics for subsequent generations

First Filial Generation (F1 generation): Mendel cross pollinated 2 plants from P generation w/ contrasting traits, offspring called F1 generation

Second Filial Generation (F2 generation): Mendel allowed the F1 generation to self-pollinate, offspring called the F2 generation

Mendel then counted his results…

Mendel’s Results F1

The recessive traits disappears

The expressed trait is said to be dominant

F2 The recessive trait

reappears!! Mendel obtained a 3:1 ratio

of dominant to recessive for each trait of the F2 generation!

Mendel proposed a Theory of Heredity Parents pass on “units of information” that operate in

the offspring to produce a trait (today we know these to be genes!)

For each characteristic there are 2 factors or alleles(1 from mom and 1 from dad) at ea/locus Homozygous - if 2 of the same alleles are inherited (true-

breeding) Heterozygous – if 2 different alleles are inherited (hybrid)

Genotype – combination of alleles an individual has Phenotype – physical appearance as a result of the

alleles inherited

Mendel’s Theory Became Laws of Heredity Law of Segregation

The members of each pair of alleles separate when gametes are formed

Law of Independent Assortment Pairs of alleles separate independently of one another

during gamete formation (only applies to genes far apart on the same chromosome or separate chromosomes)

Mendel published paper in 1866 – no interest, rediscovered in early 1900’s

Analyzing Heredity Use letters to represent alleles

Capital letters represent dominant alleles Lowercase letters represent recessive alleles Same letter designates 2 forms of the same trait

(letter of dominant trait) Ex. Tallness in pea plants

T = tall dominant allele t = short recessive allele

Genotype vs. Phenotype 2 alleles for each trait make up genotype

Genotype Phenotype

Homozygous dominant

TT Tall

Heterozygous Tt Tall

Homozygous recessive

tt Short

Probability Probability – likelihood that a specific event

will occur Probability = # of specific outcome

total # of all possible outcomes

Use this formula to predict the outcome of a genetic cross

Monohybrid Cross Monohybrid Cross - provides data about 1 pair of

contrasting traits Ex. Homozygous tall x homozygous short

Punnett Square – diagram used to predict the probable outcome of a cross

1. Write parental cross (genotypes)2. Draw box, genotype of 1 parent goes on one side, other parents

genotype on the other side3. Fill in the boxes with 1 allele from each parent to indicate possible

offspring genotypes4. Determine probability of traits5. Genotypic Ratio: homozygous dominant : heterozygous : homozygous recessive

6. Phenotypic Ratio: dominant: recessive

Test Cross Test cross is used to determine unknown

genotypes Cross unknown with a homozygous

recessive individual for that trait If ALL offspring show dominant trait, then the

unknown is homozygous dominant If any (about 1/2 ) offspring show recessive trait,

then the unknown is heterozygous

Do Now: Leslie has a long palmar muscle. Leslie has a brother, who

does not have a long palmar muscle. Leslie’s parents also lack the muscle. Leslie is married to Lamont, who does have the long palmar muscle. Their first two children are identical twin boys (Larry and Lance), who both have a long palmar muscle. Use the letters M and m to represent the alleles for this trait.

What are the genotypes of everyone in this problem? Leslie, Louis, Lamont, Larry, Lance, Leslie’s Parents

What is the most probable method of inheritance (dominant or recessive) for this trait? Explain.

Dihybrid Cross Dihybrid Cross involves 2 pairs of contrasting

traits Ex. Homozygous round yellow seeds (RRYY) x

homozygous green wrinkled seeds (rryy) Punnett Square has 16 boxes Determine possible allele combinations for each

parent and put on sides of Punnett square Fill in boxes with possible allele combinations for

offspring

Dihybrid Cross (RrYy x RrYy)

Extra Credit – Trihybrid Cross Round is dominant to wrinkled seeds Yellow seeds are dominant to green seeds Purple flower color is dominant to white flower

color Show a trihybrid cross, and use a Punnett

square to determine the phenotypic ratio for possible offspring from parents that are each heterozygous for all traits

Complex Patterns of Heredity Do not follow Mendelian Genetics

Incomplete Dominance Codominance Multiple Alleles Sex linked traits Polygenic Inheritance

Incomplete Dominance Incomplete dominance

occurs when an intermediate form of the trait is displayed in heterozygous individuals

Ex. Snapdragons

Red x White = 100% Pink!

Codominance Codominance – 2 dominant alleles are both

expressed at the same time

Ex. Roan horses

Red x White horse

= 100% Roan horse

(has both red and white hair)

Do Now: Thomas has sickle cell but his wife, Susie,

does not have sickle cell. Their daughter, Kelly has both regular cells and sickle cells.

What pattern of inheritance does sickle cell follow? How do you know?

What is the probability that Kelly and her husband Regis (who does not have sickle cell) will have a child with all normal red blood cells?

Multiple Alleles Traits with more than 2

possible alleles Ex. Blood Type (A,B,

and O) 3 possible alleles IA,IB (dominant), i (recessive)

Linked Genes Discovered by Thomas Hunt

Morgan Studied Drosophila melanogastar Crossed wildtype red-eyed female

x mutant white-eyed male Concluded white-eye mutation

linked to sex chromosome (X)

Sex-linked Traits Sex-linked traits – genes are found on the X

chromosome but not on the Y chromosome Females have 2 X chromosomes, therefore 2 alleles for

each trait and a heterozygous female would exhibit the dominant trait

Males have only 1 X chromosome, therefore only 1 allele to determine traits found on the x chromosome and will always exhibit that trait even if it is recessive

Ex. Sex-linked traits: Hemophilia, Red-Green color blindness, Male-Pattern baldness, Duchenne Muscular Dystrophy

Punnett Squares for Sex-linked Traits

Genetic Recombination Unlinked Genes - typically see 50% freq of

recombination for any 2 genes located on different chromosomes due to independent assortment of metaphase I

Linked Genes – freq of recombination varies depending on distance between linked genes due to crossing over during prophase I

Using the freq of recombination can construct a genetic map (ordered list of loci along chromosome)

Polygenic (Multi-gene Inheritance)Polygenic Inheritance – several genes influence 1 trait,

therefore we see a variety of phenotypes and a continuum from one extreme to another

Linked Genes Linked genes – genes located on the same

chromosome that tend to be inherited together

Linked genes do not always follow Mendel’s Law of Independent Assortment (he used genes on different chromosomes)

X Inactivation in Female Mammals Although female mammals inherit 2 copies of the X

chromosome, one X chromosome becomes inactivated during embryonic development and is called a Barr Body

The inactivation of an X chromosome occurs randomly in each embryonic cell, therefore females consist of a mosaic of 2 types of cells

(active x from mom or active x from dad) Ex. Tortoise shell cats

Some cells express black fur and others express orange fur

Pedigree Analysis Pedigree - diagram of

family history of a trait or disease used to study heredity

By studying a pedigree, it is possible to infer the pattern of heredity

Analyzing a Pedigree1. Determine if trait is sex-linked or autosomal

Sex-linked usually seen in males Autosomal appears in both sexes equally

2. Determine if trait is dominant or recessive If every individual w/trait has a parent w/trait then it is

dominant If individual has parents w/o trait then it is recessive

3. Determine if the trait is determined by a single gene or several

If determined by a single recessive gene, than normal parents should produce affected children with a 3:1 ratio

If determined by several genes the proportion would be much lower

Example Pedigree Ex. Pedigree 1

Pedigree 2