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Genetics

12 min read

Heredity and genes

Genetics is the study of inheritance. Characteristics are passed from parents to offspring by genes, short sections of DNA on chromosomes in the nucleus. Humans have 46 chromosomes, in 23 pairs. A gene can have different forms called alleles.

The structure of DNATwo long strands wound round each other in a double helix. Each strand has a backbone of sugar and phosphate. The strands are joined by pairs of bases: adenine always pairs with thymine and cytosine always pairs with guanine. The order of the bases along a strand is the genetic code.Base pairsA pairs with TC pairs with GA adenine T thymineC cytosine G guaninebackbone: sugar + phosphate
Dominant and recessive
A dominant allele shows its effect even when only one copy is present. A recessive allele shows only when two copies are present.
TermMeaningExample
GenotypeThe alleles of an organismTt
PhenotypeThe visible characteristicTall
HomozygousTwo identical allelesTT or tt
HeterozygousTwo different allelesTt

A monohybrid cross

Mendel studied the inheritance of one characteristic at a time, such as height in pea plants. When two heterozygous (Tt) parents are crossed, a Punnett square shows all the possible offspring.

A monohybrid crossTwo heterozygous parents, both Tt, are crossed. Each parent makes T and t gametes. The Punnett square shows the possible offspring: one TT, two Tt and one tt. Three out of four show the dominant trait and one in four shows the recessive trait, a 3 to 1 ratio.Parent 1: Tt × Parent 2: Ttgametes:TtTtTTTtTtttGenotypes1 TT : 2 Tt : 1 ttPhenotypes3 tall : 1 shortT = tall (dominant), t = short (recessive)
Genotypes in a ratio of 1 : 2 : 1, which gives 3 tall to 1 short.

Variation and cell division

Mitosis makes identical body cells for growth and repair. Meiosis makes gametes with half the number of chromosomes, so that fertilisation restores the full number. Variation arises from meiosis, random fertilisation and mutation.

Meiosis: halving the chromosome numberA diploid cell with two pairs of chromosomes copies its DNA, then divides twice. In the first division the paired chromosomes separate, giving two cells each with half the chromosome number. In the second division the two halves of each chromosome separate, giving four genetically different haploid cells, the gametes.diploid cell (2n)copy DNAchromosomes pair updivision 1two cells (n)division 2four gametes (n)Mitosis keeps the chromosome number; meiosis halves it so that fertilisation restores it.

Sex determination

Humans have 22 pairs of ordinary chromosomes and one pair of sex chromosomes: XX in a female and XY in a male. A father passes on X or Y with equal chance, so half the children are boys and half girls.

Try itChance of an affected child

Two parents are both carriers (Tt) of a recessive condition. Each child has a one in four chance of being affected. Choose the number of children.

Check yourself A tall plant (Tt) crossed with a short plant (tt). What fraction of the offspring are short?
Check yourself The genotype Tt is called...
Check yourself Which chromosomes does a boy have?

Key terms

Gene
A section of DNA that codes for a characteristic.
Allele
A different form of the same gene.
Genotype
The alleles an organism has.
Phenotype
The visible characteristic.

Summary

  • Genes on chromosomes carry heredity; alleles are dominant or recessive.
  • A Punnett square predicts the offspring: Tt × Tt gives 3 : 1.
  • Meiosis halves the chromosome number to make gametes.
  • XX is female and XY is male.
Growth Classification of Living Things