Mendelian Genetics

The Basics

Gregor Mendel??? Known as the Father of Genetics: His experiments

with Pea plants from 1856-1863 began our understanding of how traits, things like hair or eye color, height, weight, ect……., were passed down from generation to generation.

He came up with the principles of heredity that still hold true today by studying 7 characteristics of peas in his garden.

His work started and formed the base of all genetics, a field we learn more about every day.

How did He do itMendel would cross-pollinate (hybridize) two

contrasting, true-breeding pea varieties.He got true breeders by allowing self pollination.The true-breeding parents are the P generation

and their hybrid offspring are the F1 generation. Mendel allowed the F1 hybrids to self-pollinate to

produce an F2 generation. Here he came up with his 2 famous laws of segregation and independent assortment.

• Mendel reasoned that the heritable factor for white flowers was present in the F1 plants, but it did not affect flower color.

• This is where dominant and recessive come in. Dominate genes always show and ‘dominate’. Recessive genes can hide, and only show when no dominate genes are present

The Law of Segregation1. different version of genes (alleles) account for

variations in inherited characters or traits Ex Blue and Brown eyes, both eye color genes just different

versions

2. For each trait, organisms inherit 2 alleles, one from each parent.

The Law of Segregation 3. If two alleles differ, then one, the dominant allele,

is fully expressed while the other, Recessive, is masked or only partially shown (ie if you get an allele for brown eyes from your mom and an allele of blue eyes from you dad, the dominate gene (Brown) is the colour of your eyes.

Dominate alleles are capitalized, while recessive genes are non capitals

IeB- Brown eyes (dominate)b – blue eyes (recessive)

E – attached earlobes (dominate)e – unattached earlobes (recessive)

Punnett Square

Predictions

http://www.youtube.com/watch?v=V_pl5lcSUFg

Genetics Vocabulary organism with two identical alleles for a trait is

homozygous. ( TT or tt) Big letters= Dominant small or lower case = recessive.

Organisms with two different alleles for a character is heterozygous ( Tt or Pp)

http://en.wikipedia.org/wiki/List_of_Mendelian_traits_in_humans

http://faculty.southwest.tn.edu/jiwilliams/Human_Traits.htm

Genetics Vocabulary

description of an organism’s traits is its phenotype Ex) What it looks like, tall, short ,white, black ect….

description of its genetic makeup is its genotype. Ex)

Homozygous brown eyes BBGenotype – BBPhenotype – Brown Eyes Heterozygous brown eyes BbGenotype ___, Phenotype ___

Test Cross Tales the Tale

Sound Simple Right???? The relationship of genotype to phenotype is rarely

simple like in our examples because there are exceptions to all rules.

Mendel lucked out in picking peas plants because each trait is controlled by 1 gene, genetically simple. But this is rare….

some alleles show incomplete dominance where heterozygotes show a distinct intermediate phenotype, not seen in homozygotes.

Snapdragons and roses are good examples

Snapdragons

Codominance codominance 2 alleles affect the phenotype in

separate, distinguishable ways 2 dominant alleles expressed at the same time.

Ex. Is blood type. DRAW. This is also multiple allele gene have 3 alleles present. A,B,O

Fact: Just because an allele is dominant does not make it more prevalent in a population. Ex. Polydactyl is dominant to having the normal 5

fingers and toes but 399 out of 400 have the recessive genes, what we call normal 5 and 5

Cont….. most genes are pleiotropic, affecting more than one

phenotypic traitextensive symptoms of sickle-cell anemia are owed

to a single gene. epistasis, a gene at one locus, or location, alters the

phenotypic expression of a gene at a second locusEx. Mice and other mammals 1 gene determines if

there is pigment in the hair C, there is, c, is not, and another determine color B,black or b, brown. Cc is albino

Punnett Squares of Test Crosses

Homozygous

recessivea a

A

a aa

Aa Aa

aa

Homozygous recessive

a a

A

A Aa

Aa Aa

Aa

Two phenotypes

All dominant phenotype

Genetic traits in humans can be tracked through family pedigreesThe inheritance of many human traits

Follows Mendel’s laws Dominant Traits Recessive Traits

Freckles No freckles

Widow’s peak Straight hairline

Free earlobe Attached earlobe

Figure 9.8 A

Family pedigrees Can be used to determine individual genotypes

Dd Joshua Lambert

Dd Abigail Linnell

D ? John Eddy

D ? Hepzibah Daggett

D ? Abigail

Lambert

dd Jonathan Lambert

Dd Elizabeth

Eddy

Dd Dd dd Dd Dd Dd dd

Female Male Deaf Hearing

Figure 9.8 B

Parents

Offspring

Sperm

Normal

Dd

Normal

Dd

D d

Eggs

D

d

DD Normal

Dd Normal (carrier)

Dd Normal (carrier)

dd Deaf

Figure 9.9 A

Recessive Disorders Most human genetic disorders are recessive