5.2Inheritance
Inheritance, Genes & Cell Division — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions
5 min read 10 parts Lesson 1 of 9 Unit quiz
Lesson summary
5.2 Inheritance, genes and cell division: chromosomes, proteins and cell division — IGCSE BIOLOGY notes: Link chromosomes, genes, DNA and proteins, explain the inheritance of sex, and compare mitosis and meiosis.Inheritance = the transmission of genetic information from generatio…
- Sex chromosomes
- Every egg carries X
- Base sequence
- Protein shape
- DNA stays in the nucleus
- Gene expression
Part 1 of 8Chromosomes, DNA and genes
Inheritance
Inheritance the transmission of genetic information from generation to generation.
Chromosomes
- Chromosomes thread-like structures of DNA in the nucleus, carrying genetic information as genes.
Gene
Gene a short length of DNA on a chromosome that codes for a specific protein.
Alleles
Alleles different versions of the same gene.
- Humans have two copies of each chromosome, so two alleles of each gene, one from each parent.
Proteins
- Proteins may be structural (collagen), enzymes or hormones, so genes control characteristics.
Part 2 of 8Haploid and diploid nuclei
| Feature | Diploid | Haploid |
|---|---|---|
| Sets of chromosomes | Two | One |
| Human number | 46 (23 pairs) | 23 |
| Found in | Body cells and the zygote | Gametes |
Part 3 of 8Inheritance of sex
Sex chromosomes
- Sex chromosomes females are XX and males are XY.
Every egg carries X
- Every egg carries X so the father decides the sex of the child.
Sperm
- Sperm about half carry X and half carry Y.
Outcome
- Outcome an X sperm gives a girl and a Y sperm gives a boy, a 1 : 1 ratio.
Part 4 of 8Genes and protein synthesis
1Base sequence
- Base sequence the sequence of bases in a gene determines the sequence of amino acids in a protein.
2Protein shape
- Protein shape the amino acid sequence determines the shape and so the function, for example an enzyme active site.
1
The gene in the nucleus is copied (transcribed) into mRNA.
2
mRNA leaves the nucleus through a nuclear pore.
3
mRNA attaches to a ribosome in the cytoplasm.
4
The ribosome reads the bases in triplets, each coding for one amino acid.
5
tRNA brings the amino acids and they are joined in order.
6
The chain is released and folds into the protein.
- DNA stays in the nucleus
- it is too large to leave through a nuclear pore.
Part 5 of 8Which proteins are made
Gene expression
- Gene expression whether a gene is transcribed and translated in a particular cell.
All body cells
- All body cells contain the same genes, but most are switched off in any one cell.
Only needed proteins
- Only needed proteins only genes for proteins vital to the cell are switched on.
Examples
- Examples red blood cell precursors make haemoglobin; pancreatic cells make insulin.
Part 6 of 8Mitosis
Mitosis
Mitosis nuclear division giving rise to genetically identical cells.
Replication
- Replication each chromosome is copied exactly before division.
Separation
- Separation the copies are pulled apart, maintaining the chromosome number.
Uses
- Uses growth, repair of damaged tissue, replacement of cells and asexual reproduction.
Stem cells
- Stem cells unspecialised cells that divide by mitosis to give daughter cells that can specialise.
- Embryonic stem cells come from the ball of cells formed when the zygote divides.
Part 7 of 8Meiosis and comparison
Meiosis
- Meiosis a reduction division in which the chromosome number halves from diploid to haploid, giving genetically different cells.
Site
- Site testes and ovaries in humans; anthers and ovules in plants.
Variation
- Variation new combinations of maternal and paternal chromosomes plus random fertilisation make every offspring different.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Divisions | One | Two |
| Cells produced | Two | Four |
| Chromosome number | Diploid | Haploid |
| Genetic make-up | Identical to parent and each other | Different from each other and the parent |
Part 8 of 8More from the specification
More from the specification
Genotype and phenotype
- Genotype and phenotype genotype is the combination of alleles in an individual, phenotype is the observable characteristic that results
Dominant and recessive
- Dominant and recessive a dominant allele shows in the phenotype even when one copy is present, a recessive allele only when two copies are present
Homozygous and heterozygous
- Homozygous and heterozygous identical alleles at a gene locus versus two different alleles
Punnett square
- Punnett square grid showing the possible gametes of the parents and the expected ratio of offspring genotypes, such as 3:1 in a monohybrid cross
Genotype and phenotype
- Genotype and phenotype genotype is the combination of alleles in an individual, phenotype is the observable characteristic that results
Dominant and recessive
- Dominant and recessive a dominant allele shows in the phenotype even when one copy is present, a recessive allele only when two copies are present
Homozygous and heterozygous
- Homozygous and heterozygous identical alleles at a gene locus versus two different alleles
Punnett square
- Punnett square grid showing the possible gametes of the parents and the expected ratio of offspring genotypes, such as 3:1 in a monohybrid cross
Blood group
- A phenotype set by alleles I-A, I-B and I-O.
Protein synthesis
- Making proteins from amino acids using the DNA code.
Monohybrid inheritance
- Inheritance of one gene.
Recessive
- Expressed only when no dominant allele is present.
Dominant
- Expressed when present in the genotype.
Genotype
- The alleles an organism has.
Homozygous
- Having two identical alleles.
Phenotype
- The observable characteristic.
Heterozygous
- Having two different alleles.
Probability
- The chance that an offspring has a given genotype or phenotype.
Punnett square
- A grid predicting offspring genotypes.
Key terms
| Term | Meaning |
|---|---|
| Gene | A length of DNA that codes for a protein. |
| Allele | A version of a gene. |
| Diploid | Having two sets of chromosomes. |
| Haploid | Having one set of chromosomes. |
| mRNA | Messenger RNA that carries a gene copy to the ribosome. |
| Gene expression | Whether a gene is transcribed and translated in a cell. |
| Mitosis | Nuclear division giving genetically identical cells. |
| Meiosis | Reduction division giving genetically different haploid cells. |
Avoid these mistakes
1
Saying genes and alleles are the same — an allele is one version of a gene.
2
Saying the mother decides the sex of the child — the sperm carries X or Y.
3
Saying DNA leaves the nucleus to make proteins — a copy, mRNA, leaves.
4
Saying cells have different genes — they have the same genes but express different ones.
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