Mendelian Genetics
Introduced| English | Meaning |
|---|---|
| gene/dʒiːn/ | A DNA sequence that contributes to a functional product, such as a protein or RNA molecule. |
| alleles/əˈliːlz/ | Alternative versions of a gene found at the same locus on a chromosome. |
| dominant/ˈdɒmɪnənt/ | Under complete dominance, this allele determines the heterozygote's phenotype. |
| recessive/rɪˈsesɪv/ | Describing an allele whose effect is masked by a dominant allele in a heterozygote. |
| genotype/ˈdʒenətaɪp/ | The genetic constitution of an organism, or the alleles it has at specified loci. |
| phenotype/ˈfenətaɪp/ | The observable characteristics of an organism, resulting from its genotype and environmental influences. |
| Punnett square/ˈpʌnɪt skweə/ | A grid used to predict possible offspring genotypes from combinations of parental gametes. |
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…
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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…
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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).
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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?
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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.
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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.
Write from a probability of 1/4
- Both parents are Tt.
- T is dominant, and the cross is a monohybrid.
What is the probability of a tt offspring?
Two heterozygous parents give one recessive homozygote in four.
What is the chance of tt from Tt times Tt?
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The tt box is one of the four equally likely boxes.