Unit 8 — Genetic Probability and Inheritance

BIO.3B

Unit 8: Need to Know and Be Able to Do

A gene is an instruction for a trait; an allele is a version of that gene.
Genotype is the allele pair an organism carries. Phenotype is the observable trait.
Homozygous means two matching alleles; heterozygous means two different alleles.
With complete dominance, one dominant allele produces the dominant phenotype. A recessive phenotype requires two recessive alleles.
Each offspring receives one allele for each gene from each parent.

Translate between genotype descriptions and letter notation.
Determine the phenotype produced by a genotype and list the possible genotypes for a phenotype.
Explain how two parents with dominant phenotypes can have an offspring with a recessive phenotype.

A monohybrid cross tracks one trait in a 2×2 Punnett square.
Each parent contributes one allele to each possible offspring genotype.
Every box represents a possible outcome, not a guaranteed child; each box in a 2×2 square is 25%.
Genotype questions count exact allele pairs. Phenotype questions group genotypes that look alike.

A dihybrid cross tracks two traits and may use a 4×4 square with 16 boxes.
A heterozygous parent such as HhBb can produce four gametes: HB, Hb, hB, and hb.
Independent assortment means the alleles for different genes are inherited independently.
The 9:3:3:1 phenotype ratio applies only when both parents are heterozygous for both independently assorting traits.
The product rule multiplies separate probabilities to find the chance that two outcomes occur together.

In incomplete dominance, the heterozygote has a blended phenotype.
In codominance, both alleles are fully expressed in the heterozygote.
Incomplete dominance and codominance usually produce three phenotypes and a 1:2:1 phenotype ratio in a heterozygous cross.
ABO blood type has three alleles: I^A, I^B, and i. I^A and I^B are codominant; i is recessive.

Distinguish complete dominance, incomplete dominance, and codominance from an offspring phenotype.
Predict genotype and phenotype ratios for non-Mendelian crosses.
Determine possible ABO genotypes, offspring blood types, and parent genotypes.

An X-linked gene is carried on the X chromosome; the Y usually has no matching allele.
Females have two X chromosomes and may be unaffected, carriers, or affected. Males have one X and are either unaffected or affected.
A son receives his X chromosome from his mother. A daughter receives one X from each parent.
X-linked recessive traits appear more often in males because one recessive allele on their only X is expressed.

Write and interpret X-linked genotypes using superscripts.
Complete a sex-linked Punnett square and calculate risk for all children, sons, or daughters.
Use family outcomes to identify carriers and determine which parent supplied an allele.

In a pedigree, a square represents a male, a circle represents a female, an open symbol is unaffected, and a solid symbol is affected.
A half-shaded symbol represents a carrier only when the pedigree key defines it that way.
A recessive trait can skip generations and can appear in a child of two unaffected carriers. A dominant trait usually appears in every generation.
An X-linked recessive pedigree usually shows more affected males and no father-to-son transmission.

Read a pedigree key and identify relationships, generations, affected individuals, and carriers.
Determine whether a pattern is dominant, recessive, autosomal, or X-linked.
Assign possible genotypes and use them in a Punnett square to calculate disease risk.

Unit 8 Flashcards

Study 164 cards by lesson deck or as one whole-unit deck.

Unit 8 Interactive Review

Practice 48 questions per run, sampled from the complete 84-question Unit 8 review bank.

Unit 8 Review Quiz

Complete a 30-question graded review quiz sampled from 60 whole-unit questions.

8.1 Alleles, Genotype, Phenotype

Practice alleles, genotypes, phenotypes, dominance, and inheritance vocabulary.