The Genetic Basis of Inheritance

From Chromosomes to Genes

Chromosomal Basis of Inheritance

• Traits are transmitted by chromosomes which contain units of heredity called genes

• Genes are formed from DNA

Mendel and the History of Genetics

• Gregor Mendel • Born 1822• Began his work at

age 21• A monk and school

teacher interested in plant breeding

• Studied pea plants• Discovered the basic

principles of heredity

Mendel’s Studies• Pea plants were an

ideal choice for study • Displayed seven traits

in one of two contrasting forms:– seed shape, seed color,

seed coat color, pod shape, pod color, flower position, stem length

• Mendel studied 30,000 plants in 7 years– Kept careful records &

looked for mathematical patterns

Pure Lines• Mendel recognized the 2 contrasting forms

as distinct varieties, or pure lines • In a pure line, the offspring have same trait

as the parents• ‘Tall’ always produced ‘tall,’ etc.• Called parental pure lines "P1" generation • First generation of offspring are F1 (for

filial), then F2 and so on• Hybrid = An offspring produced by breeding

2 pure lines • What happens if you cross 2 pure lines?

Dominance• Mendel crossed the two pure lines for

each trait.• These were the P1 generation. • All offspring of these crosses (F1) had

the trait of only one parent• The trait of the other parent

disappeared in the F1 generation (but reappeared in F2)

• Mendel hypothesized that there were 2 factors for each trait

• Mendel called 1 factor dominant because it prevailed

Dominant vs. Recessive Traits

• Mendel crossed F1 plants with other F1 plants, producing the F2 generation

• Not only did the recessive trait reappear in the F2

generation, but in a consistent proportion:– 1/4 showed the recessive trait (ratio 3:1)

• Mendel referred to the factor that was hidden in the F1 as recessive

• If an offspring carries 2 dominant or one dominant and one recessive factor, the offspring will appear to have the dominant trait.

• If the offspring carries 2 recessive factors, it will appear to have the recessive trait

The Experiment

• P Generation: – Cross 2 pure lines

– – One white, one

purple• F1 are all purple

– Purple is dominant• Cross F1 x F1 (all

purple)– ¼ F2 offspring are

white– White is recessive

Principle of Segregation

• Members of each pair of genes separate, or segregate, when gametes are formed.– Recall what we now know happens to

chromosomes during meiosis

• From Mendel:– For each characteristic an individual carries

2 ‘factors’– Each parent contributes one of its 2 factors

to each offspring. – Chances of contributing either factor are

equal.

Genes and Alleles

• We now know that the heredity units are genes.• The separate forms of a gene that Mendel

called ‘factors’ = alleles• One allele in a pair may prevent the other from

being expressed• If an offspring carries 2 dominant, or one

dominant and one recessive allele, the offspring will appear to have the dominant trait.

• If an offspring carries 2 recessive alleles, it will appear to have the recessive trait.

Each individual carries one copy (allele) of a gene on the chromosome from their mother, and a second copy on the homologous chromosome from their father.

Representing Genes & Alleles

• Generally the capitalized first letter of the dominant trait is used to represent the dominant allele

• The recessive allele is then lower case of the same letter (e.g. T for tall, so t for short)

• Each characteristic can be represented by a pair of letters representing the genes

• Some multi-allele systems (more than 2 possible forms of the trait) use different conventions

• Traits carried on sex chromosomes are written as a superscript of the X or Y chromosome 

Hereditary Terminology

• Genes – The units of heredity• Alleles – The separate forms of a gene that

Mendel called factors • Genotype - The set of alleles for a characteristic

possessed by an organism• Phenotype - The appearance of the organism; the

trait that is actually expressed • Homozygous - The 2 alleles in a pair are identical

– An individual can be either homozygous dominant (2 identical dominant alleles) or homozygous recessive (2 identical recessive alleles)

• Heterozygous – The 2 alleles in a pair are different

Genotype vs. Phenotype

Punnett Squares• A way to visualize

crosses • Punnett square can be

used to determine probability of different genotypes or phenotypes

• Each box contains a possible combination of alleles for offspring

• Punnett square can be used to determine probability of different genotypes or phenotypes

• Mendel’s F1 cross can be shown as a punnett square

• Shows 3:1 ratio

Determining Genotype• If you know the phenotype, is it

possible to determine the genotype?• If an organism shows the recessive

trait, you know the genotype.– It must be homozygous recessive (tt)

• What if it shows the dominant trait? • If the dominant trait is expressed

there are 2 possibilities: • homozygous dominant ( TT)• or heterozygous (Tt)

Test Crosses• Cross the unknown with a homozygous

recessive (tt) x ?– This is a test cross (or backcross)

• If the unknown is TT:– then crossing with tt yields all Tt – All offspring are tall– all have same phenotype and the same genotype

• If the unknown is genotype Tt, when crossed with tt – 1/2 of the offspring will be Tt and 1/2 will be tt– so 1/2 will show a different phenotype, making it

easy to tell

Demonstrating a Test Cross

Incomplete Dominance• In all of Mendel's cases, 1 allele was clearly

dominant– This is not always the case

• Some alleles show incomplete dominance• Heterozygotes express traits that are a

blend of the phenotypes of the 2 alleles – red and white flowers make pink

• Still obey the law of segregation• Only difference is phenotype of

homozygous dominant individual is different from phenotype of heterozygous individual

Demonstrating Incomplete Dominance

Co-Dominance

• Occurs when both alleles for a gene are expressed in a heterozygote

• Example – Red & white produce roan color in some animals because both genes are expressed

Multi-allele Systems• Some traits are the result of more than 2

possible alleles at a locus• The ABO blood group is an important

example• 3 possible alleles:

– IA - produces A antigen; anti-B antibodies in blood serum

– IB - produces B antigen; anti-A antibodies in serum– i - produces no antigen; anti-A & anti-B antibodies in serum

• IA & IB are co-dominant alleles• i is recessive

The ABO Blood System

Working with 2 Alleles• Crosses so far have examined only 1

characteristic = monohybrid crosses• It is possible to consider more than 1

characteristic at a time– Crosses involving 2 genes = dihybrid crosses– Three genes = trihybrid

• Mendel crossed plants that differed in 2 characteristics

• This led to Law of Independent Assortment

Law of Independent Assortment• Mendel showed that traits produced by

dominant factors do not necessarily appear together

• 2 or more pairs of genes segregate independently of one another during the formation of gametes– Genes are distributed to gametes independently

• We now know this is true: – recall independent assortment of chromosomes from

meiosis

• If 2 traits are carried on the same chromosome independent assortment does not hold– If 2 traits are located close together on the same

chromosome they are linked

Dihybrid Crosses• Involve 2 genes• Can demonstrate independent assortment• Cross homzygous dominant for 2 alleles with

homozygous recessive for 2 alleles:– Y = yellow – R = round

• YYRR x yyrr can produce:– Yellow/ round– Yellow/ wrinkled– Green/ round– Green wrinkled

• Produces 9:3:3:1 ratio

Diagramming a Dihybrid Cross

Sex Linkage• Autosomal trait

– a gene carried on a non-sex chromosome & present in 2 copies

• Sex linked trait – a gene carried on a sex chromosome that is

present in both sexes; one copy in one sex, 2 in the other

– X chromosome in mammals– Z chromosome in birds (one copy in females, 2 in

males)

• Y- linked trait – a gene carried on the Y chromosome in humans;

very rare

Showing Sex Linkage• Symbols are written as superscript of the

sex chromosome:– Xa - X chromosome carrying the recessive allele– XA – X chromosome carrying the dominant allele– No superscript is used for the normal or wild

type allele

• Suspect sex linkage when the ratios of phenotypes are different in males and females

Pedigrees• Used to determine mode of inheritance

when few individuals, but several generations are involved

• Assume genetic trait discussed is rare, so individuals marrying into the family are not assumed to carry the trait

• Symbols:O female shaded = affected male partial shading = carrier

Pedigree Analysis

Polygenic Inheritance

• Most traits are not limited to 2 possibilities, yes or no

• Most traits are a continuum (quantitative)

• Examples: height, skin color

• 2 or more genes act additively on a trait

Pleitropy

• Genes have multiple phenotypic effects

• The same gene that produces dark pigment in skin might produce dark pigment in fur

• Pleiotropic Advantage:• Some genes that have one negative

effect, also convey a benefit – Example: sickle cell trait may have

provided the heterozygote an advantage against malaria

Sickle Cell Trait & Pleiotropy

Epistasis

• A gene at one locus alters expression of a second gene

• Example: If black (B) is dominant over brown (b) in mice, but . . .

• A second gene (C) determines if fur pigment is made at all,

• Then you must be (CC) or (Cc) before the mice can be either black or brown

Diagramming Epistasis