inheritance patterns

Heredity

a biological process in which genetic info, physical traits, and characteristics are passed down from parent to offspring

Gregor Mendel’s study

• Determined how heredity biologically works

• He manually pollinated (bred) different pea plants together and studied their offspring + used careful mathematical counting of traits

Mendel’s Laws of Inheritance (3)

Law of dominance, Law of segregation, Law of Independent Assortment

Law of Dominance

in a heterozygous individual (Aa), the dominant allele will ALWYAS override/hide the presense of the recessive allele

Law of Segregation

during the formation of gametes (meiosis), the two allels from each gene sperate/segregate from each other, resulting in each gamete recieveing one allele from each parent

EX: AA x aa —> Aa

Law of Independent Assortment

The possible alleles of different genes get sorted into gametes INDEPENDENTLY from one another (one inherited trait does not dictate the inheritance of another)

homozygous, heterozygous, true-breeding

Homozygous = all recessive or dominant (AA, aa)

Heterozygous = recessive + dominant (Aa)

True-breeding = EVERYTHING is recessive or dominant; inheritance will always be the same trait (EX: AA x AA = AA or aa x aa = aa)

P, F_1, F₂

P = Parental Generation (original parents)

F₁ = First Generation

F₂ = Second Generation

Modes of Inheritance

Incomplete Dominance, Codominance, Sex-Linked

Incomplete Dominance

• a heterozygous phenotype, mixture of two homozygous phenotypes

• alleles are blended/mixed (neither is completely dominant or recessive)

• EX: Black dog (HH) + White dog (H’H’) = Grey dog (H’H)

Codominance

• A heterozygous phenotype, separately expresses both alleles

• Both alleles are seen as their own (dominant but not mixed)

• EX: Black dog (HH) + White dog (H’H’) = Spotted Dog (H’H)

Sex-Linked

• traits that are passed down on the X or Y chromosomes (focus on X-linked recessive alleles)

(Sex-Linked) X-Linked Recessive

• People with XX (female) chromosomes need two copies of the allele to express the trait

  • IF heterozygous: they are considered “carriers” (ONLY FEMALES)

• People with XY (male) chromosomes only need one copy of the allele to express the trait (more common one)

Carriers

they do not express the trait but have the ability to pass it on

XX, X’X’, X’X (for females)

XX = does not have the trait

X’X’ = does have the trait (DASH MEANS “PRIME”)

X’X = carrier (DASH MEANS “PRIME”)

Expected phenotypic ratios for dihybrid crosses (2)

Heterozygous x Heterozygous (AaBb x AaBb) = 9:3:3:1

Heterozygous x Homozygous recessive (AaBb x aabb) = 1:1:1:1

Phenotypic ratio orders (2)

dominant : recessive

dominant-domaint : domiant-recessive : recessive-dominant : recessive-recessive

Polygenic Inheritance VS. Pleiotropy

Polygenic Inheritance:

• when a single phenotypic trait is controlled by two or more genes combining/working together

• shows vast differences among populations (EX: differing heights)

Pleiotropy:

• when a single phenotypic trait is controlled by one single gene that has mutated to have mutliple effects

Testcrosses (func, genotypes used)

• Breeding two individuals

• To determine the unknown genotype of an organism that shows a dominant phenotype (finding out the second unknown allele)

• Unknown individual (usually with a dominant allele) × homozygous recessive

Pedigrees (func)

• to trace how a trait/genetic disorder is passed down/inherited, to determine carriers, to predict the probability of future children inheriting a condition

Autosomal recessive traits

• If BOTH parents express the trait = ALL offspring express the trait

• An affected child can have two unaffected parents (carriers)

• males + females affected equally

Autosomal dominant traits

• Affected individuals have AT LEAST ONE affected parent

• affected parents that are heterozygous can have an unaffected child

X-Linked recessive traits

• Affected mothers = ALL sons are affected

• No father-to-son transmission

• Affected daughters require an affected father

• More males are affected

X-Linked dominant traits

• Affected fathers = ALL daughters are affected

• No father-to-son transmission

• Both males + females affected