Basic Genetics
FoundationGenes are DNA segments that control traits, and alleles come in dominant and recessive forms; with genotype, phenotype, homozygous and heterozygous concepts plus the Punnett square you can predict offspring ratios such as 3:1, a test cross reveals an unknown genotype, and the sex chromosomes XX and XY decide gender with even odds.
- A gene is a DNA segment controlling a trait, and paired genes at the same position on homologous chromosomes are alleles.
- Dominant alleles show even in heterozygotes while recessive ones show only when homozygous; genotype is the internal make-up, phenotype the outward trait.
- The law of segregation: alleles separate as gametes form, so an F1 self-cross gives an F2 with phenotypes at 3:1 and genotypes at 1:2:1.
文體小知識：科學知識說明文——用孟德爾豌豆雜交引入等位基因、顯隱性和 Punnett 方格，解釋性狀怎樣按固定比例遺傳，並說明性染色體怎樣決定性別。
§Text Passage
1Heredity is the passing of traits from parents to offspring, and the basic unit controlling a trait is the gene — a segment of DNA carrying genetic information. Chromosomes come in pairs, and the paired genes at the same position are alleles: the pea gene for tall stems and the gene for short stems are alleles of each other. Alleles are dominant or recessive, written as capital and small letters respectively (T and t).
2The genotype is the combination of genes an individual carries — homozygous (TT or tt) or heterozygous (Tt) — while the phenotype is the trait actually observed. TT and Tt plants both look tall because the dominant allele masks the recessive one; only tt shows the short form. Mendel's pea crosses, analysed with careful counting, gave rise to the law of segregation and founded genetics.
3Cross a pure tall pea with a pure short one and all the F1 offspring are heterozygous tall; self-pollinating the F1 gives an F2 in which the traits segregate, tall to short in a 3:1 ratio. A Punnett square, listing every egg and sperm combination cell by cell, verifies the ratio. In a test cross, an individual showing the dominant trait is crossed with a homozygous recessive one, and the offspring reveal its genotype.
4Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Females are XX and males are XY. Every egg carries an X, while sperm carry an X or a Y in equal numbers; fertilisation by an X sperm gives a girl and by a Y sperm a boy, so it is the sperm that decides, with even odds each time.
5Individuals of the same species differ — variation. Continuous variation, such as height, spreads across a range and depends on many genes plus the environment; discontinuous variation, such as blood group, falls into clear categories. Its sources include genetic information, environmental factors and mutation — the reason family members look different, and the raw material for the later topic of evolution.
1Summary 重點摘要
Genes are DNA segments that control traits, and alleles come in dominant and recessive forms; with genotype, phenotype, homozygous and heterozygous concepts plus the Punnett square you can predict offspring ratios such as 3:1, a test cross reveals an unknown genotype, and the sex chromosomes XX and XY decide gender with even odds.
2Vocabulary cards Vocabulary
A segment of DNA that carries genetic information and controls a particular trait.
Different forms of a gene at the same position on a pair of homologous chromosomes, such as the tall and short stem genes.
An allele expressed even in the heterozygote, conventionally written as a capital letter.
An allele expressed only when homozygous, masked by the dominant allele and written as a small letter.
The combination of alleles an individual carries for a trait, such as TT, Tt or tt.
The trait actually shown by an individual, determined by genotype together with the environment.
A genotype with identical alleles (TT or tt); pure-breeding organisms are homozygous.
A genotype with different alleles (such as Tt), showing the dominant trait but capable of passing on the recessive one.
A grid listing each parent's gametes in rows and columns to predict the genotypes and phenotypes of offspring.
The chromosome pair that determines sex: XX in females and XY in males.
3Step by step Steps
- Step 1: Genes and allelesChromosomes in the nucleus are made of DNA, and a segment of DNA controlling a trait is a gene — for instance, the gene for stem height in peas. In human body cells the 23 pairs of chromosomes each contain one from the father and one from the mother; paired genes at the same position controlling the same trait are alleles. For example, the pea stem-height gene exists as tall (T) and short (t) forms, so any pea plant has a genotype of TT, Tt or tt.
- Step 2: Dominant, recessive, genotype and phenotypeAlleles differ in strength: a dominant allele works whenever present, while a recessive one shows only when homozygous. Genotypes TT and Tt both give tall stems, and only tt gives short — so one phenotype can hide several genotypes. For example, two tall pea plants look identical, yet one is pure-bred (TT) and the other hybrid (Tt); appearance alone cannot tell them apart, which is exactly why a test cross is needed. Genotype is the internal combination, phenotype the outward result — never mix the two in answers.
- Step 3: Mendel's experiment and the law of segregationMendel crossed pure-bred peas: tall (TT) with short (tt), and every F1 plant was tall, showing tall is dominant; self-pollinating the F1 gave an F2 where tall and short appeared together, always in a 3:1 ratio. From this he proposed the law of segregation: paired alleles separate when gametes form, each gamete receiving only one. For example, a Tt plant makes T-carrying and t-carrying gametes in equal halves, and random fertilisation lets the ratio emerge naturally — the most basic rule in genetics.
- Step 4: How to draw a Punnett squareThe Punnett square is the standard tool for predicting crosses, drawn in three steps: write both parents' genotypes, list the gametes each can produce, then combine them cell by cell. Take the F1 self-cross Tt × Tt: gametes T and t each make up half, and the four cells give TT, Tt, Tt and tt — a genotype ratio of 1:2:1 and a phenotype ratio of tall to short at 3:1. For example, in exams fill in all four cells before counting ratios to avoid misses; the square itself is the full reasoning and carries the method marks.
- Step 5: The test cross — reading a hidden genotypeAn individual showing the dominant trait may be homozygous or heterozygous, and the eye cannot tell. The test cross solves it: mate it with a homozygous recessive partner (tt) and read the offspring. If all offspring show the dominant trait, the parent is pure-bred (TT); if the offspring split about 1:1, the parent is heterozygous (Tt). For example, to check whether a tall pea is pure, a breeder crosses it with a short one: all-tall offspring mean pure stock, while half short means a hybrid to discard before it spoils the line.
- Step 6: Sex determinationHuman chromosome pair 23 is the sex chromosomes, deciding sex: females are XX and males XY. Every egg from the mother carries an X; sperm from the father carry an X or a Y in equal halves. An egg fertilised by an X sperm develops as a girl (XX) and by a Y sperm as a boy (XY), each with a chance of one half. For example, draw a Punnett square crossing the mother's X with the father's X and Y: two cells give XX and two give XY — it is the type of sperm that decides, and favouring boys has no scientific basis whatsoever.
- Step 7: Variation and its sourcesIndividuals of the same species vary. Continuous variation spreads over a range with no clear categories, such as height and body mass, shaped by many genes plus the environment; discontinuous variation falls into distinct classes, such as blood group and tongue-rolling, set by one or a few genes. Sources include recombination of genetic information, environmental factors such as nutrition and light, and mutation. For example, identical twins share every gene, yet one who trains daily and one who sits all day drift apart in build and health — clear evidence of the environment shaping the phenotype.
4Diagrams Diagrams
5Worked example Example
In peas, tall stem (T) is dominant over short stem (t). A pure tall pea is crossed with a pure short pea to give the F1, and the F1 is self-pollinated to give the F2. (1) State the genotype and phenotype of the F1. (2) Draw a Punnett square to account for the 3:1 ratio in the F2. (3) If a tall F2 plant is crossed with a short plant, state the possible phenotype ratios of the offspring.
- The parents are TT × tt: the pure tall produces T gametes and the pure short produces t gametes, so every F1 plant is Tt — phenotype all tall.
- The F1 self-cross Tt × Tt: each parent makes T and t gametes, and the four cells of the Punnett square give TT, Tt, Tt and tt — a genotype ratio of 1:2:1.
- Counting phenotypes: TT, Tt and Tt are all tall while tt is short, so tall to short is 3:1, matching Mendel's data.
- Among the tall F2 plants, one third are TT and two thirds are Tt: TT × tt gives all tall offspring; Tt × tt gives 1 tall : 1 short.
- Answer: All F1 plants are Tt and phenotypically tall. Selfing the F1 (Tt × Tt) gives genotypes TT:Tt:tt at 1:2:1 and phenotypes tall:short at 3:1. Crossing tall F2 plants with short ones: a TT parent gives all-tall offspring while a Tt parent gives 1 tall : 1 short. In genetics answers, always show the gametes, the square and the ratios, and label each ratio as genotype or phenotype.
6Common mistakes Mistakes
7Quick check Quick check
Q1A tall pea crossed with a short one gives offspring half tall and half short. What is the tall parent's genotype?
Q2Two heterozygous tall peas (Tt × Tt) are crossed. What is the chance of a short offspring?
Q3A couple already have two daughters. What is the chance the third child is a boy?
Q4Distinguish genotype from phenotype, giving one example of each.
Q5What is a test cross, and how do the results reveal the parent's genotype?
8Recap Recap
- A gene is a DNA segment controlling a trait, and paired genes at the same position on homologous chromosomes are alleles.
- Dominant alleles show even in heterozygotes while recessive ones show only when homozygous; genotype is the internal make-up, phenotype the outward trait.
- The law of segregation: alleles separate as gametes form, so an F1 self-cross gives an F2 with phenotypes at 3:1 and genotypes at 1:2:1.
- The Punnett square combines gametes cell by cell — the standard way to predict crosses — and a test cross reveals a dominant individual's genotype.
- Sex chromosomes are XX in females and XY in males; sperm decide sex with even odds, and variation comes as continuous or discontinuous, from genes, environment and mutation.