GENETICSGENETICS

Mendel and Mendel and the Gene the Gene IdeaIdea

GeneticsGenetics

The study of The study of heredityheredity..

Gregor Mendel (1860’s)Gregor Mendel (1860’s) discovered the discovered the fundamental fundamental principles principles ofof genetics genetics by by breedingbreeding garden peasgarden peas..

He Used Plants that looked like this:

True Breeding

Plants:

consistently

have

offspring with

same

trait as parent.

GeneticsGeneticsAllelesAlleles

1.1. Alternative forms ofAlternative forms of genes.genes.

2.2. Units that determine Units that determine heritable traits.heritable traits.

Dominant AllelesDominant Alleles

Dominant alleles Dominant alleles

((TTTT - - tall pea plantstall pea plants))

a. homozygous a. homozygous dominantdominant

Homozygous = two alleles that are Homozygous = two alleles that are the same. the same.

Recessive AllelesRecessive Alleles

Recessive alleles Recessive alleles ((tt tt - - dwarf pea plantsdwarf pea plants))

a. homozygous recessivea. homozygous recessive

Homozygous = two of the same allelesHomozygous = two of the same alleles

Recessive = Hidden allele when Recessive = Hidden allele when dominant is present.dominant is present.

Mixed AllelesMixed Alleles

HeterozygousHeterozygous ( (TtTt - - tall pea plantstall pea plants))

Mixed set of alleles. Dominant takes Mixed set of alleles. Dominant takes over for physical trait (phenotype).over for physical trait (phenotype).

PhenotypePhenotype

Outward appearanceOutward appearance Physical characteristicsPhysical characteristics

Examples:Examples:

1.1. tall pea planttall pea plant

2.2. dwarf pea plantdwarf pea plant

GenotypeGenotype Arrangement of genes that produces the Arrangement of genes that produces the

phenotypephenotype Example:Example:

1.1. tall pea planttall pea plantTT = tall TT = tall (homozygous dominant)(homozygous dominant)

2.2. dwarf pea plantdwarf pea planttt = dwarf tt = dwarf (homozygous (homozygous

recessive)recessive)3.3. tall pea planttall pea plant

Tt = tall Tt = tall (heterozygous)(heterozygous)

Punnett squarePunnett square

A A Punnett square Punnett square is used to show the is used to show the possible possible combinationscombinations of of gametesgametes..

Showing a cross with Pea Showing a cross with Pea PlantsPlants

tall (TT) vs. dwarf (tt) pea tall (TT) vs. dwarf (tt) pea plantsplants

t

t

T T

tall (TT) vs. dwarf (tt) pea tall (TT) vs. dwarf (tt) pea plantsplants

t

t

T T

Tt

Tt

Tt

Tt All Tt = tall(heterozygous tall)

produces the11stst generation generation

Cross the Cross the 11stst generation generation

tall (Tt) vs. tall (Tt) pea tall (Tt) vs. tall (Tt) pea plantsplants

T

t

T t

tall (Tt) vs. tall (Tt) pea plantstall (Tt) vs. tall (Tt) pea plants

TT

Tt

Tt

tt

T

t

T t

produces the22ndnd generation generation

1/4 (25%) = TT1/2 (50%) = Tt1/4 (25%) = tt

1:2:1 genotype1:2:1 genotype 3:1 phenotype3:1 phenotype

Monohybrid CrossMonohybrid Cross

Mendel did A breeding Mendel did A breeding experiment that tracks the experiment that tracks the inheritance of a inheritance of a single single trait.trait.

Principle of Segregation

Segregation

The principle of segregation is explained by the behavior of homologous chromosomes at meiosis.

Law of Independent Law of Independent AssortmentAssortment

Genes for different traits are Genes for different traits are separated and distributed to separated and distributed to gametes (sex cells) independently of gametes (sex cells) independently of one anotherone another

Incomplete DominanceIncomplete Dominance This is when both This is when both

alleles contribute to the alleles contribute to the phenotype of a phenotype of a heterozygous individual heterozygous individual to produce a trait that to produce a trait that is not exactly like either is not exactly like either parents.parents.

Ex:Ex: A flower has genes for A flower has genes for white petals (W) and red white petals (W) and red petals (R).petals (R). When you mix When you mix white flowers with red white flowers with red flowers you get pink flowers you get pink flowers.flowers.

CodominanceCodominance Two dominant alleles Two dominant alleles

are expressed at the are expressed at the same time.same time.

The human ABO blood The human ABO blood group illustrates group illustrates another genetic another genetic phenomenon – phenomenon – codominance.codominance.

The AB phenotype The AB phenotype (genotype I(genotype IAA and I and IBB is is an example.an example.

Polygenic Inheritance – When a Single Trait is Influenced by Many Genes

Height is a polygenic trait

Multiple AllelesMultiple Alleles

Many genes are Many genes are present in 3 or more present in 3 or more versions (alleles) – this versions (alleles) – this is known as multiple is known as multiple alleles. alleles.

The human ABO blood The human ABO blood group is determined group is determined by three alleles (IA, IB, by three alleles (IA, IB, and i) of a single gene.and i) of a single gene.

Pleiotropy – When One Allele Influences Many Traits

Sex-Linked TraitsSex-Linked Traits

Traits carried on sex chromosomes X or YTraits carried on sex chromosomes X or Y Ex: Red green color blindness, HemophiliaEx: Red green color blindness, Hemophilia

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