Biology 102Biology 102

Patterns of Inheritance (cont.)Patterns of Inheritance (cont.)

Lecture outlineLecture outline Inheritance of multiple traitsInheritance of multiple traits

Same chromosome (linkage)Same chromosome (linkage)Sex chromosomesSex chromosomes

OverviewOverviewSex linkageSex linkage

Incomplete dominanceIncomplete dominanceCodominanceCodominanceOther variationsOther variations

Sickle cell anemia exampleSickle cell anemia examplePedigreesPedigrees

Crosses with two traitsCrosses with two traits If genes are on different chromosomes, they If genes are on different chromosomes, they

sort independentlysort independently

Genes on the same chromosome Genes on the same chromosome tend to be inherited togethertend to be inherited together

They are on the same chromosome. They are on the same chromosome. Considered to be “linked”.Considered to be “linked”.

Recombination creates new Recombination creates new combination of “linked” allelescombination of “linked” alleles

The farther away two genes are on a chromosome The farther away two genes are on a chromosome from each other, the more likely they will be from each other, the more likely they will be exchanged during crossing overexchanged during crossing over

The closer together two genes are on a The closer together two genes are on a chromosome, the more likely they will travel chromosome, the more likely they will travel together together

Sex chromosomesSex chromosomes Differ in appearanceDiffer in appearance Only small parts are Only small parts are

homologous (carrying same homologous (carrying same genes)genes)

Sex-linked genesSex-linked genes Found only on one of the sex Found only on one of the sex

chromosomeschromosomes Only 20 genes on YOnly 20 genes on Y

Related to male characteristicsRelated to male characteristics 1500 genes on X1500 genes on X

Related to a variety of traits (few Related to a variety of traits (few are gender-related)are gender-related)

Sex determinationSex determination

Male determines Male determines the gender in the gender in mammalsmammalsHow so?How so?

Birds are the Birds are the oppositeoppositeFemales XY, Females XY,

Males XXMales XX

Sex-linked inheritance: Sex-linked inheritance: Eye color in fruit fliesEye color in fruit flies

Incomplete dominanceIncomplete dominance

Snapdragon exampleSnapdragon example Heterozygote is Heterozygote is

intermediate between the intermediate between the two homozygotestwo homozygotes R codes for enzyme R codes for enzyme

catalyzing production of red catalyzing production of red pigmentpigment

R’ codes for non-functional R’ codes for non-functional enzymeenzyme

Amount of enzyme Amount of enzyme determines amount of determines amount of pigmentpigment

Human eye color:Human eye color:Polygenic, incomplete dominancePolygenic, incomplete dominance

CodominanceCodominance

Both phenotypes expressed fully.Both phenotypes expressed fully. Example: Human blood groupsExample: Human blood groups

These genes code for enzymes that These genes code for enzymes that stick sugars to cell-surface glycoproteinsstick sugars to cell-surface glycoproteins

Alleles A and B code for addition of Alleles A and B code for addition of distinct sugarsdistinct sugars

O does not code for a sugarO does not code for a sugar

Make antibodies to sugars not Make antibodies to sugars not present on own cellspresent on own cells

10%10%

40%40%

46%46%

4%4%

B or ABB or AB

A or ABA or AB

O,AB,O,AB,A,BA,B

(universal)(universal)

ABAB

B or OB or O

A or OA or O

OO

AB, A,AB, A,B, OB, O

(universal)(universal)

AA

BB

BothBoth

NeitherNeither

BB or BOBB or BO

AA or AOAA or AO

OOOO

ABAB

OO

ABAB

BB

AA

FreqFreqDonatesDonatesRe-Re-

ceivesceives

Anti-Anti-bodiesbodies

RBCsRBCsGenotypeGenotypeTypTyp

ee

Human Blood TypesHuman Blood Types

Other variationsOther variationsEnvironmental influencesEnvironmental influences

Expression of gene for black pigment in Expression of gene for black pigment in Himalayan rabbit depends upon temperatureHimalayan rabbit depends upon temperature

Most traits affected by both genes and Most traits affected by both genes and environmentenvironment

PleitropyPleitropyMultiple effects of a single geneMultiple effects of a single geneExample: Sickle cell anemiaExample: Sickle cell anemiaAlso great example from single error in a single Also great example from single error in a single

nucleotide to whole organism effectnucleotide to whole organism effect

Some human genetic diseasesSome human genetic diseases

Sickle-cell anemia: A case of Sickle-cell anemia: A case of incomplete dominanceincomplete dominanceGo from gene to whole organism effectsGo from gene to whole organism effects

Sickle cell anemia: From amino acid Sickle cell anemia: From amino acid substitution to whole body effects…substitution to whole body effects…

A A single nucleotide is substituted in 18th position of B-chain of Hb molecule (6th amino acid) GUG instead of GAG

Altered primary protein structure Valine instead of Glutamic acid

Altered tertiary structure Altered quaternary protein

structure Chains don’t fit together

correctly

Sickle cell anemia: From amino acid Sickle cell anemia: From amino acid substitution to whole body effects…substitution to whole body effects…

Hemoglobin molecules stick togetherRed blood cell shape is altered

Sickle cell anemia (cont.)Sickle cell anemia (cont.) Whole body effects (symptoms of sickle cell anemia)

Investigating human genetic disordersInvestigating human genetic disorders

Pedigrees: Use to figure out how trait is inherited Pedigrees: Use to figure out how trait is inherited (dominant, recessive, sex-linked)(dominant, recessive, sex-linked)