Genetics and Heredity

Grade 10 · Science · Study Guide

Understand how traits are inherited from parents to offspring through genes, alleles, and the principles Gregor Mendel discovered.

Genetics is the study of heredity — how traits are passed from parents to offspring through genes. Gregor Mendel, working with pea plants in the 1860s, discovered the fundamental principles of inheritance. He found that traits are controlled by discrete factors (now called genes), which come in pairs (alleles). He proposed two laws: the Law of Segregation (each parent passes one allele for each gene to offspring, chosen randomly) and the Law of Independent Assortment (alleles for different genes are inherited independently of each other — for genes on different chromosomes).

Alleles can be dominant (expressed when one or two copies are present, denoted by capital letters) or recessive (only expressed when two copies are present, denoted by lowercase letters). An organism's genotype describes its allele combination (e.g., Tt), while the phenotype is the observable trait (tall). Homozygous means both alleles are the same (TT or tt); heterozygous means they differ (Tt).

A Punnett square is a tool for predicting the probability of offspring genotypes and phenotypes. Beyond simple dominance, some traits show incomplete dominance (heterozygote has a blended phenotype), codominance (both alleles are fully expressed), or are controlled by multiple genes (polygenic inheritance). Sex-linked traits are carried on the X or Y chromosome, explaining why some conditions like color blindness are more common in males.

Genotype vs. Phenotype Examples (Pea Plant Height: T=tall, t=short)
Genotype	Homozygous/Heterozygous	Phenotype
TT	Homozygous dominant	Tall
Tt	Heterozygous	Tall (T is dominant)
tt	Homozygous recessive	Short

Key terms

Gene: A segment of DNA that codes for a specific trait; found at a specific location (locus) on a chromosome.
Allele: A variant form of a gene; organisms typically have two alleles for each gene (one from each parent).
Dominant allele: An allele that is expressed in the phenotype whenever it is present (even in one copy).
Recessive allele: An allele that is only expressed in the phenotype when two copies are present (homozygous recessive).
Punnett square: A grid used to predict the probability of different genotype and phenotype combinations in offspring.
Incomplete dominance: Inheritance pattern where the heterozygote shows a phenotype intermediate between the two homozygous phenotypes.

Flashcards

Genotype: The genetic makeup of an organism, describing the specific alleles it carries (e.g., Tt, TT, tt).
Phenotype: The observable characteristics of an organism resulting from its genotype and environment.
Homozygous: Having two identical alleles for a gene (e.g., TT or tt).
Heterozygous: Having two different alleles for a gene (e.g., Tt); also called a carrier in some contexts.
Law of Segregation: Mendel's first law: each parent contributes one of its two alleles to each offspring, randomly.
Codominance: Both alleles are fully and simultaneously expressed in the heterozygote (e.g., AB blood type).

Practice questions

A homozygous tall plant (TT) is crossed with a short plant (tt). What fraction of offspring will be tall?
1. 0%
2. 25%
3. 50%
4. 100%
Show answer
Answer: 100%
TT × tt: all offspring get one T allele from the TT parent and one t from the tt parent, making all offspring Tt (heterozygous tall). T is dominant, so all are tall.
A recessive allele is expressed in the phenotype only when the organism is homozygous recessive (tt).
Show answer
Answer: True
A recessive allele is 'masked' by the dominant allele in heterozygotes. It is only expressed when the organism inherits two copies — the homozygous recessive genotype.
An organism's observable characteristics are called its ___.
Show answer
Answer: phenotype
Phenotype refers to observable traits (height, eye color, blood type), which result from the genotype interacting with environmental factors.
Two heterozygous tall plants (Tt × Tt) are crossed. What is the probability of a short offspring?
1. 0%
2. 25%
3. 50%
4. 75%
Show answer
Answer: 25%
Tt × Tt Punnett square: TT (25%), Tt (50%), tt (25%). Only tt offspring are short — 25% probability.
Incomplete dominance produces a blended phenotype in the heterozygote.
Show answer
Answer: True
In incomplete dominance, neither allele is fully dominant. The heterozygote shows an intermediate phenotype (e.g., red flower × white flower → pink flower in snapdragons).

Fun facts

Mendel published his groundbreaking work in 1866, but it was largely ignored for over 30 years until other scientists rediscovered it in 1900.
Humans have about 20,000–25,000 genes, but our traits are far more complex than simple dominant/recessive pairs — most are polygenic.
Identical twins have the same genotype, but differences in gene expression (epigenetics) mean they can develop different traits over time.

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