Unit 5: Heredity

Genetics

The scientific study of heredity

Allele

Refers to the different versions of a gene.

Genotype

• genetic makeup.

  • Use letters (alleles).

Phenotype

•  physical trait. 

  • Not always visible (e.g. enzymes).

Recessive

• must inherit 2 copies to produce phenotype. 

  • lower case

Dominant

• will produce a phenotype over other alleles

Homozygous

means same

Heredity

the passing on of genetic characteristics

Gene

segment of DNA that codes for a protein/trait.

Homologous chromosomes

Chromosomes that have the same sequence of genes, but not necessarily the same versions, or alleles, of those genes. 

Wild Type

• references which trait is most common in the population

  • Typically non-mutant alleles (common)

Genome

collection of ALL of an organism’s genes.

Heterozygous/hybrid

Means different; two different alleles are inherited

Why do two alleles get inherited for each gene?

The offspring inherits one allele from each parents, resulting in 2 of either the same, or one of each.

Law of Dominance

Law of Independent Assortment

Alleles of genes on homologous chromosomes assort independently during gamete formation

Law of Segregation

The two alleles for each gene separate during gamete formation

Monohybrid P generation genotype

• parent generation

• Homozygous dominant and homozygous recessive are crossed

Monohybrid F₁ generation genotype

• 100% are heterozygous

Monohybrid F₂ generation genotype

• homozygous dominant

• heterozygous

• homozygous recessive

Monohybrid P generation phenotype (single trait)

• # 1 : # 1

Phenotypic ratio

• a way to report the resulting phenotypes of a cross

• # dominant : # recessive

Monohybrid F₁ generation phenotype

4:0 (1:0)

Monohybrid F₂ generation phenotype

3:1

The testcross

Organism with dominant phenotype x organism with recessive must use this to discover the truth about its genotype

• Recessive offspring = unknown was heterozygous

• Only dominant offspring = inconclusive
  

Two Trait Phenotypic Ratio

#1: Dom/Dom

#2: Dom/Rec

#3: Rec/Dom

#4: Rec/Rec

Dihybrid P generation genotype

• One Dom/Dom allele

• One Rec/Rec allele

Dihybrid F₁ generation genotype

• heterozygous- Dom; Rec/ Dom; Rec

Dihybrid F₂ generation genotype

#1: Dom/Dom

#2: Dom/Rec

#3: Rec/Dom

#4: Rec/Rec

Incomplete Dominance

• No true dominant/recessive trait

The result: BOTH SHOW UP (blended) in a heterozygote

Codominance

• No true dominant/recessive trait.

• In heterozygotes, two phenotypes appear (pattern).

  • No mixed/blended phenotype results

Multiple Alleles

• More than 2 alleles for a gene exist in a population

Polygenic

• A single trait is determined by many genes

Number of genes involved in Codominance, Incomplete dominance, and Multiple Alleles

1

Number of traits involved in Codominance, Incomplete dominance, and Multiple Alleles

1 (t)

Number of alleles involved in Codominance and Incomplete Dominance

2

Number of alleles involved in Multiple Alleles

3 or MORE

Number of traits in Polygenic Inheritance

2+ genes

Polygenic Inheritance special characteristics

• Bell curve characteristic of polygenic traits

• Continuous range of variation

• Most fall within the intermediate phenotypes

Human examples of polygenic inheritance

• Height (more than 400 gene regions)

• Skin tone (many)

• Hair color (many - 2 main genes, possibly over 20)

• Eye color (2 main, at least 50! others***)

Y Chromosome

• Few genes 

• Sex-determining region (SRY) gene

  • Helps form testes which trigger testosterone release

X Chromosome

• ~900+ genes

• Mutations could result in: 

  • Hemophilia (blood disorder)

  • Muscular dystrophy

  • Color-blindness

Disorders linked to the X chromosome are called

X-linked

• More than 60 disorders are X-linked

Genotypes for X-Linked Recessive

• Affected XX = hom rec; 

• Affected XY = one rec allele.

• Unaffected XX = heterozygous (carriers) or hom dom; 

• Unaffected XY = one dom allele.

Which sex is more likely to be affected by X-Linked Recessive Disorders?

• XY = have a higher chance of inheritance. 

Only have one X chromosome

Genotypes for X-Linked Dominant

• Affected XX = heterozygous or hom. dom; 

• Affected XY = one dom allele. 

• Unaffected XX = hom rec; 

• Unaffected XY = one rec allele. 

Who is affected by X-linked dominant disorders?

• XX have a higher chance of being affected

• Inherit two X-chromosomes - higher chance of getting the allele. 

Y Linked Disorders

Exclusively affect individuals with a Y chromosome