Grade 10-11 (age 15-17)

high school Human Genetics

Unlike peas, people cannot be crossed at will, so inheritance is worked out from pedigrees, charts of a trait across several generations of a family. Where a pattern shows up in the pedigree depends on whether the gene is on an autosome or on the X chromosome, and whether the trait is dominant or recessive. If two unaffected parents have an affected child, the trait is recessive, and X-linked recessive traits show up more often in males, who have only one X chromosome. The diagram steps through three cases, autosomal dominant, autosomal recessive and X-linked recessive, showing the genotypes of the parents and children and the share of children who show the trait.

Why Human Inheritance Is Hard to Study

👨‍👩‍👧‍👦 Features of Human Inheritance

①People cannot be crossed experimentally, generations are long and families are small, so inheritance is studied through pedigrees and twin studies

②Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males), and a gene is inherited differently depending on which kind it sits on

③Traits such as height and skin color, shaped by many genes together with the environment, vary continuously

Pedigree Analysis

Inheritance patternAutosomal dominant

💡 What to Look for in the Three Pedigrees

①Autosomal dominant (Aa × aa): half of the children of an affected parent are affected, with no difference between the sexes

②Autosomal recessive (Aa × Aa): even when both parents are unaffected carriers, a quarter of the children are affected, and the chance rises when close relatives marry

③X-linked recessive (XᴬY × XᴬXᵃ): sons get their only X from the mother, so half of the sons are affected; daughters get Xᴬ from the father and are not affected, but half of them are carriers

Comparing Inheritance Patterns

Inheritance Patterns

Autosomal vs X-Linked Inheritance

Item	Autosomal Dominant	Autosomal Recessive	X-linked recessive
Gene location	autosome	autosome	X chromosome
Shows up when	one A is enough	aa (homozygous recessive)	XᵃY (male) or XᵃXᵃ (female)
Sex difference	none	none	shows up more often in males
Examples	Huntington's disease, polydactyly	albinism, sickle cell anemia	red-green color blindness, hemophilia

Other Patterns of Human Inheritance

Inheritance Types

Patterns of Human Inheritance

Worked Examples

Example 1

A father with blood type A (genotype I^A i) and a mother with blood type B (genotype I^B i) have children. List every blood type the children could have.

The father's gametes carry I^A or i, and the mother's carry I^B or i.

Fertilization gives I^A I^B (AB), I^A i (A), I^B i (B) and ii (O), each with probability 1/4.

▸ all four: A, B, AB and O

When each parent carries one i, type A and type B parents can still have a type O (ii) child. When I^A and I^B occur together, both show, giving type AB.

Example 2

A mother who carries red-green color blindness (X-linked recessive), X^A X^a, and an unaffected father, X^A Y, have children. What is the chance of color blindness in their sons, and in their daughters?

Daughters always get X^A from the father, so they are X^A X^A or X^A X^a and none is color blind.

Sons get Y from the father and an X from the mother, so X^A Y (unaffected) and X^a Y (color blind) each have probability 1/2.

▸ sons 1/2, daughters 0

These are the same parents as in the X-linked recessive pedigree. Half of the sons are affected, and the daughters, though unaffected, are carriers half the time, so the trait can reappear in their sons.

Summary

Reading a Pedigree

both parents unaffected but a child affected → recessive / both parents affected but a child unaffected → dominant

sort out dominant or recessive first, then autosomal or X-linked

Testing for X-Linked Recessive

every son of an affected mother is affected / the father of an affected daughter is always affected

if even one person breaks these rules, the trait is not X-linked recessive

CSAT-style problem, adapted

A mother who carries red-green color blindness (X^A X^a) and a color-blind father (X^a Y) have a child. What is the probability that the child is color blind?

①0

②14

③12

④34

⑤1

▸ ③ 12

Daughters get X^a from the father, so they are X^A X^a (carrier) or X^a X^a (color blind): half of the daughters are color blind. Sons are X^A Y (unaffected) or X^a Y (color blind): half of the sons are color blind.

Half of the sons and half of the daughters are color blind, so the probability for any child is also 1/2.

🎯 Exam Points

①In pedigree analysis, decide dominant or recessive first, then autosomal or X-linked

②If unaffected parents have an affected child, the trait is recessive, and for an autosomal recessive trait both parents are Aa

③For an X-linked recessive trait, half of a carrier mother's sons are affected

④ABO blood type has the alleles Iᴬ and Iᴮ (codominant) and i (recessive), giving 6 genotypes and 4 phenotypes

⑤Polygenic traits vary continuously and are strongly affected by the environment

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Last updated 2026-10-17

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