Mechanisms of Evolution: Mechanisms of Evolution: Mendelian GeneticsMendelian Genetics

Anthropology 101Anthropology 101Hybrid CourseHybrid Course

Mendelian GeneticsMendelian Genetics The appearance of an individual is derived The appearance of an individual is derived

from the cell divisions we just describedfrom the cell divisions we just described Traits are maintained in the organism through Traits are maintained in the organism through

mitosismitosis They are transmitted from parent to offspring They are transmitted from parent to offspring

through meiosisthrough meiosis When haploid cells are united between couples to When haploid cells are united between couples to

become diploidbecome diploid Each parent contributes exactly half their makeup to Each parent contributes exactly half their makeup to

their offspringtheir offspring

Individual Genetics; :ConceptsIndividual Genetics; :Concepts Traits Traits are inherited through the chromosomes and are inherited through the chromosomes and

their constituent genestheir constituent genes The genetic composition of a trait is known as a The genetic composition of a trait is known as a

genotypegenotype A A phenotypephenotype is a trait of a genotype that is visible or is a trait of a genotype that is visible or

otherwise observable and can be measuredotherwise observable and can be measured HomozygousHomozygous traits are those with two identical traits are those with two identical

alleles in a gene pairalleles in a gene pair HeterozygousHeterozygous traits are those with two different traits are those with two different

alleles in a gene pairalleles in a gene pair

Individual Genetics: When Individual Genetics: When Genotypes Become PhenotypesGenotypes Become Phenotypes

A A dominantdominant allele is one whose trait appears in allele is one whose trait appears in both homozygous heterozygous combinationboth homozygous heterozygous combination

A A recessiverecessive allele is one whose trait appears allele is one whose trait appears only in homozygous combination.only in homozygous combination.

A A codominant codominant allele is one whose trait reflects allele is one whose trait reflects the genetic combination of two different allelesthe genetic combination of two different alleles

Punnett squaresPunnett squares illustrate how these principles illustrate how these principles workwork

Case Study of Monogenic Trait: Case Study of Monogenic Trait: Tasters vs. NontastersTasters vs. Nontasters

Most of us can taste the bitterness of Brussels Most of us can taste the bitterness of Brussels sproutssprouts

This is the ability to taste phenylthiocarbamide This is the ability to taste phenylthiocarbamide (PTC) (PTC)

Tasters are dominant; nontasters are recessiveTasters are dominant; nontasters are recessive A Punnett Square allows us to determine the A Punnett Square allows us to determine the

proportion of tasters vs. nontasters of PTC proportion of tasters vs. nontasters of PTC This is a table that gives us a visual count of This is a table that gives us a visual count of

the allele for each trait. the allele for each trait.

PTC Tasters and Nontasters: PTC Tasters and Nontasters: Generation IGeneration I

Suppose one parent is a taster and the other is Suppose one parent is a taster and the other is a nontaster in the first generationa nontaster in the first generation

All the offspring (Generation II) will be tasters All the offspring (Generation II) will be tasters in the second generation, as shown in the next in the second generation, as shown in the next panel. panel.

To simplify, we use only a 2 X 2 tableTo simplify, we use only a 2 X 2 table

Punnett Square of Tasters and Non-Punnett Square of Tasters and Non-Tasters: Generation IITasters: Generation II

t (nontaster) t (nontaster)t (nontaster) t (nontaster)

T (taster) Tt TtT (taster) Tt Tt

T (taster) Tt TtT (taster) Tt Tt

PHC Tasters and Nontasters: PHC Tasters and Nontasters: Generation IIGeneration II

The second generation generate a new The second generation generate a new combination of phenotypescombination of phenotypes

The proportion is now 1 homozygote for PTC The proportion is now 1 homozygote for PTC tasters, 1 homozygote for nontasters, and 2 tasters, 1 homozygote for nontasters, and 2 heterozygotes for tasters/nontasters.heterozygotes for tasters/nontasters.

Punnett Square of Tasters and Non-Punnett Square of Tasters and Non-Tasters: Generation IIITasters: Generation III

T (tastersT (tasters t (nontasters)t (nontasters)

T TTT TT TtTt

t Tt ttt Tt tt

Codominant GenesCodominant Genes Some alleles are codominant: one trait does Some alleles are codominant: one trait does

not trump the othernot trump the other A species of flower, four-o’clocks, may come A species of flower, four-o’clocks, may come

in red and whitein red and white Their hybrids thus come in pink in Generation Their hybrids thus come in pink in Generation

IIII In the Punnett Square, in Generation III, the In the Punnett Square, in Generation III, the

proportion is 1:2:1 (one red, two pinks, and proportion is 1:2:1 (one red, two pinks, and one white)one white)

Mendel’s Laws: Law of Mendel’s Laws: Law of SegregationSegregation

Mendel found that there were three principles Mendel found that there were three principles of inheritance resulting from the study of pea of inheritance resulting from the study of pea plants for seven characteristics.plants for seven characteristics.

In so doing, he found that traits of the parent In so doing, he found that traits of the parent generation do not blend in those of their generation do not blend in those of their offspring.offspring.

Rather, one gene for each trait is Rather, one gene for each trait is segregated segregated from other genes for other traits. from other genes for other traits.

SegregationSegregation: Separation of alleles in the formation of : Separation of alleles in the formation of gametes (sex cells)gametes (sex cells)

Mendel’s Law: Law of Independent Mendel’s Law: Law of Independent AssortmentAssortment

Independent Assortment: Independent Assortment: differing traits are differing traits are inherited independently of each other (genes on inherited independently of each other (genes on separate chromosomes)separate chromosomes)

For example whether a pea plant flower is violet or For example whether a pea plant flower is violet or white is separated from smooth or wrinkled peaswhite is separated from smooth or wrinkled peas

In other words, a white flowering plant can yield In other words, a white flowering plant can yield either a wrinkled or a smooth pea; so can a violet either a wrinkled or a smooth pea; so can a violet flowering plantflowering plant

So flower color is independent from smoothness of So flower color is independent from smoothness of peaspeas

The PHC assortments in the Punnett Squares are The PHC assortments in the Punnett Squares are another example of this law.another example of this law.

Mendel’s Laws: Law of Mendel’s Laws: Law of RecombinationRecombination

Though independent, genes can recombined to Though independent, genes can recombined to allow further genetic varietyallow further genetic variety

In meiosis, some genes cross over, allowing In meiosis, some genes cross over, allowing even further variety. even further variety.

LinkagesLinkages Nevertheless, if alleles occur on the same Nevertheless, if alleles occur on the same

chromosome, they will be inherited together.chromosome, they will be inherited together. Sex-linked traits are one example: secondary Sex-linked traits are one example: secondary

characteristic are linked to the primary characteristic are linked to the primary characteristics (organs of reproductioncharacteristics (organs of reproduction

ConclusionConclusion Genes are ultimately responsible for our traitsGenes are ultimately responsible for our traits Cells are maintained by cell division called Cells are maintained by cell division called

mitosismitosis Our traits are passed down from one Our traits are passed down from one

generation to the next by meiosisgeneration to the next by meiosis Through meiosis, we can predict what traits Through meiosis, we can predict what traits

will be passed down and in what proportion. will be passed down and in what proportion.