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Biology Unit 5: Heredity

Meiosis makes gametes with half the chromosomes, and the way alleles separate explains how traits are inherited.

Covers: Meiosis, Mendel's laws, Punnett squares, non-Mendelian inheritance, pedigrees

Study guide

Meiosis makes gametes (sex cells) with half the chromosome number: one diploid cell divides twice to produce four genetically different haploid cells. Variation comes from crossing over between homologous chromosomes in prophase I, independent assortment of homologous pairs in metaphase I, and random fertilization. Errors such as nondisjunction cause chromosome disorders like Down syndrome.

Mendel's experiments with pea plants established the basic laws. Genes come in versions called alleles; an organism inherits one allele from each parent. Dominant alleles mask recessive ones. The genotype is the allele combination (AA, Aa or aa), and the phenotype is the observable trait. The law of segregation says the two alleles separate into different gametes; the law of independent assortment says genes on different chromosomes are inherited independently.

Punnett squares predict offspring ratios. A monohybrid cross Aa × Aa gives a 1 : 2 : 1 genotype ratio and a 3 : 1 phenotype ratio. A dihybrid cross AaBb × AaBb gives a 9 : 3 : 3 : 1 phenotype ratio. A test cross with a homozygous recessive reveals an unknown genotype. Probability rules (multiply for "and," add for "or") handle multiple genes.

Non-Mendelian inheritance includes incomplete dominance (blending, such as pink flowers from red and white), codominance (both alleles show, such as AB blood type), multiple alleles, polygenic traits (height, skin color), linked genes that stay together unless crossing over separates them, and sex-linked traits carried on the X chromosome, which show up more in males because they have only one X.

Pedigrees trace traits through families. Recessive traits can skip generations, while dominant traits appear in every generation that carries them.

In meiosis I, homologous chromosomes pair up, cross over, and then separate to opposite poles, which cuts the chromosome number in half (diploid to haploid). In meiosis II, sister chromatids separate, much as they do in mitosis. Each of the four resulting cells has one copy of each chromosome. Nondisjunction happens when homologs (in meiosis I) or sister chromatids (in meiosis II) fail to separate, so some gametes get an extra copy of a chromosome (n + 1) and others get none (n − 1). Fertilization then produces a trisomic or monosomic zygote. The extra or missing chromosome changes how much of each of its genes' products the cell makes.

An organism with an odd number of chromosome sets, such as a triploid (3n), can't pair its homologs evenly in meiosis. Its gametes get random, unbalanced numbers of chromosomes, so the organism is usually sterile. That's why seedless bananas and watermelons are triploid. A chi-square test checks whether observed results differ significantly from the expected ratio: χ² = Σ(observed − expected)²/expected. The null hypothesis states that there is no real difference. If χ² is larger than the critical value for the degrees of freedom (number of categories − 1), usually at p = 0.05, the null hypothesis is rejected.

Key ideas

• Meiosis produces 4 haploid, genetically different cells.
• Crossing over and independent assortment create variation.
• Law of segregation: each parent passes one allele per gene.
• Dominant alleles mask recessive ones in heterozygotes.
• Non-Mendelian patterns: incomplete dominance, codominance, sex-linked and polygenic traits.

Formulas and rules

Common mistakes

• Mixing up genotype (letters) and phenotype (appearance).
• Forgetting that males have one X, so X-linked recessive traits show up more in males.

Words to know

Meiosis

Cell division that makes 4 genetically different haploid cells.

Genotype vs phenotype

Genotype is the alleles; phenotype is the observable trait.

Monohybrid cross ratio

Aa × Aa gives a 3 : 1 phenotype ratio.

Codominance

Both alleles show fully, like AB blood type.

X-linked recessive trait

Shows more in males, who have only one X.

Haploid vs. diploid

One set of chromosomes (n) vs. two sets (2n).

Crossing over

Homologous chromosomes swap segments in prophase I.

Independent assortment

Homologous pairs line up randomly, so genes on different chromosomes sort independently.

Nondisjunction

Chromosomes fail to separate, causing an abnormal number.

Allele

A version of a gene.

Law of segregation

The two alleles separate into different gametes.

Dihybrid cross ratio

AaBb × AaBb gives 9 : 3 : 3 : 1.

Test cross

Crossing with a homozygous recessive to reveal an unknown genotype.

Incomplete dominance

A blended phenotype, like pink from red and white.

Polygenic trait

A trait controlled by many genes, like height.

Pedigree

A family chart tracing a trait through generations.

Mendel's laws

Segregation (alleles separate) and independent assortment (genes sort independently).

Punnett square

A grid that predicts offspring genotypes from the parents' alleles.

Non-Mendelian inheritance

Patterns like incomplete dominance, codominance, multiple alleles, polygenic and sex-linked traits.

Sample practice questions

21 more questions like these, from level 1 to 5, are in the practice set.