In pea plants, purple flower color is dominant to red flower color and long pollen grains

1. Gametes and reproduction:

(i) Sexual reproduction is the reproduction involving the fusion of gametes (fertilisation) to produce a zygote.

(ii) A gamete is a sex cell; during sexual reproduction, two gametes fuse together to form a zygote; gametes are usually haploid.

(iii) Fertilisation is the fusing of the nuclei of two gametes, to form a zygote.

(iv) The zygote is a cell formed by the fusion of the nuclei of two gametes; most zygotes are diploid.

(v) Diploid cells contain two complete sets of chromosomes; can be signified by the symbol 2n.

(vi) Haploid cells contain one complete set of chromosomes; can be signified by the symbol n.

(vii) Homologous chromosomes are pairs of chromosomes in a diploid cell that have the same structure and the same genes at the same loci, but not necessarily the same varieties of those genes.

(viii) Meiosis is the nuclear division that results in the production of four daughter cells with half the chromosome number of the parent cell and with reshuffled alleles; in animals and plants, it results in the formation of gametes.

(ix) Crossing over is the exchange of alleles between non-sister chromatids of homologous chromosomes during meiosis I.

(x) Meiosis consists of two divisions.

(a) The first division, meiosis I, is a reduction division that separates the homologous chromosomes so that each cell now has only one of each pair. 

(b) The second division, meiosis II, separates the chromatids of each chromosome. Meiotic division, therefore, produces four cells, each with one complete set of chromosomes.

(xi) The cells produced by meiosis are genetically different from each other and from their parent cell. This results from an independent assortment of the chromosomes as the bivalents line up on the equator during metaphase I, and also from crossing over between the chromatids of homologous chromosomes during prophase I. 

(xii) Reduction division is the nuclear division that results in a reduction in chromosome number; the first division of meiosis is a reduction division.

2. The production of genetic variation:

(i) Locus is the position of a gene on a chromosome.

(ii) Genetic variation also results from random fertilisation, as gametes containing different varieties of genes fuse together to form a zygote.

(iii) Independent assortment is the production of different combinations of alleles in daughter cells, as a result of the random alignment of bivalents on the equator of the spindle during metaphase I of meiosis.

3. Genetics:

(i) An organism's genetic constitution is its genotype. 

(ii) An organism’s observable characteristics are its phenotype, which is influenced by the expression of its genes. 

(iii) Different varieties of a gene are called alleles. 

(iv) Alleles may show dominance, codominance or recessiveness. 

(v) An organism possessing two identical alleles of a gene is homozygous; an organism possessing two different alleles of a gene is heterozygous. 

(vi) If a gene has several different alleles, such as the gene for human blood groups, these are known as multiple alleles.

4. Monohybrid inheritance and genetic diagrams:

(i) Monohybrid crosses consider the inheritance of one gene.

(ii) The genetic diagram is a standard format in which the results of a genetic cross are predicted and explained.

(iii) Punnett square is a part of a genetic diagram in which the genotypes of the offspring are worked out from the genotypes of the gametes.

(iv) The genotype of an organism showing dominant characteristics can be determined by looking at the offspring produced when it is crossed with an organism showing recessive characteristics. This is called a test cross. 

(v) A gene found on the X chromosome but not on the Y chromosome is known as a sex-linked gene. 

(vi) Genes that are close together on a chromosome that is not a sex chromosome are said to be autosomally linked.

5. Dihybrid inheritance:

(i) Dihybrid inheritance is the inheritance of two genes.

(ii) Epistasis is the interaction of two genes at different loci; one gene may affect the expression of the other.

(iii) Autosomal linkage is the presence of two genes on the same autosome, (any chromosome other than a sex chromosome) so that they tend to be inherited together and do not assort independently.

(iv) Crossing over breaks the linkage between genes on the same chromosomes.

6. The chi-squared (χ²) test:

The chi-squared (χ²) test can be used to find out whether any differences between expected results and observed results of a genetic cross are due to chance or whether the difference is significant.

7. Genes, proteins and phenotype:

(i) The HBB gene codes for the beta-globin polypeptides in haemoglobin. 

(ii) An allele of HBB gene with a different base sequence produces sickle cell haemoglobin. 

(iii) The allele of the HTT gene that is responsible for Huntington's disease includes a repeated triplet of nucleotides called a 'stutter'. 

(iv) Albinism and haemophilia show the effect on the phenotype of missing or inactive polypeptides.

8. Control of gene expression:

(i) A gene is said to be expressed when it is transcribed to mRNA and then the mRNA is translated to produce a protein.

(ii) The lac operon provides an example of how a prokaryote can alter the transcription of a cluster of structural genes coding for enzymes concerned with lactose uptake and metabolism, depending on whether or not lactose is present. 

(iii) 'Structural' genes code for the proteins required by a cell for its structure or metabolism, whereas 'regulatory' genes control the expression of other genes. 

(iv) A repressor protein can block the synthesis of a 'repressible' enzyme, by binding to the gene's operator site. 

(v) An 'inducible' enzyme is synthesised only when its substrate is present.

(vi) A transcription factor is a protein that binds to DNA and affects whether or not a gene is transcribed.

(vii) Transcription factors in eukaryotes make sure that genes are expressed in the correct cell, at the correct time and to the correct extent. 

(viii) In plants, gibberellins allow gene transcription by causing the breakdown of DELLA proteins which inhibit the binding of transcription factors.

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