Mendelian Genetics
Introduced| English |
|---|
| gene/dʒiːn/ |
| alleles/əˈliːlz/ |
| dominant/ˈdɒmɪnənt/ |
| recessive/rɪˈsesɪv/ |
| genotype/ˈdʒenətaɪp/ |
| phenotype/ˈfenətaɪp/ |
| Punnett square/ˈpʌnɪt skweə/ |
Predicting what offspring inherit
- A tall pea plant crossed with a short one — what do the offspring look like?
- In the 1860s, a monk named Mendel worked out the rules.
- His simple patterns still predict inheritance today.
- With them, you can forecast a cross before it even happens.
Genes and alleles
- A gene 基因 is an instruction for a trait, like flower colour.
- Each gene comes in versions called alleles 等位基因.
- A dominant 显性 allele shows even with a single copy (written with a capital, A).
- A recessive 隐性 allele shows only with two copies (written lower-case, a).
An allele is…
An allele is one version of a gene — for example, a version for brown eyes or blue eyes.
Genotype and phenotype
- The genotype 基因型 is the pair of alleles an organism carries (AA, Aa, or aa).
- The phenotype 表型 is the trait you can actually see.
- Both AA and Aa show the dominant phenotype; only aa shows the recessive one.
- So the same phenotype can hide different genotypes.
A dominant allele is one that…
A dominant allele shows in the phenotype with even one copy; a recessive one needs two.
The set of alleles an organism carries is its ____ (its genetic make-up).
The genotype is the alleles present; the phenotype is the trait you can see.
The Punnett square
- A Punnett square 庞纳特方格 charts every way the parents' alleles can combine.
- Put one parent's alleles along the top, the other's down the side.
- Fill each box with one allele from each — those are the possible offspring.
- Reading the boxes gives the ratio of offspring types.
Predict the offspring
Aa × Aa → 3 : 1
Pick the parents' genotypes and read the offspring ratios straight from the Punnett square.
Crossing two Aa parents gives what ratio of dominant to recessive offspring?
Aa × Aa gives AA, Aa, Aa, aa — three showing the dominant trait to one recessive: a 3 : 1 ratio.
Select all true statements about Mendelian genetics.
A recessive trait needs two recessive alleles to show. The other three are correct.
A recessive trait appears only when both alleles are recessive (aa). Just one recessive allele (Aa) is hidden by the dominant one. So an organism can carry a recessive allele without ever showing it — a "carrier".
Aa × Aa, box by box:
- The four boxes come out AA, Aa, Aa, aa.
- Three of them (AA, Aa, Aa) show the dominant trait; one (aa) shows the recessive.
- That is the famous 3 : 1 ratio Mendel found again and again.
A gene comes in versions called alleles — dominant (shows with one copy) or recessive (needs two). The genotype is the alleles present; the phenotype is the visible trait. A Punnett square combines the parents' alleles to predict offspring ratios, like the 3 : 1 from Aa × Aa.