INHERTITENCE Flashcards
Inheritance
The transmission of genetic information from generation to generation
chromosome
A thread-like structure of DNA carrying genetic information in the form of genes
gene
A length of DNA that codes for a protein
allele
A version of a gene
mitosis
Nuclear division that gives rise to genetically identical cells
Meiosis
Nuclear division that gives rise to cells that are genetically different
genotype
The genetic make-up of an organism in terms of the alleles present
phenotype
The observable features of an organism
homozygous
Having two identical alleles of a particular gene
heterozygous
Having two different alleles of a particular gene
dominant allele
An allele that is expressed if it is present
recessive
An allele that is expressed only when there is no dominant allele of the gene present
haploid nucleus
23
diploid
46
meiosis reproduction definition
Reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
sex linked characteristic
A characteristic in which the gene responsible is located on a sex chromosome, which makes it more common in one sex than the other
Inheritance of sex in humans

the genetic code
• Each nucleotide carries one of four bases (A, T, C or G). A string of nucleotides therefore holds a sequence of bases. This sequence forms a code, which instructs the cell to make particular proteins. Proteins are made from amino acids linked together (Chapter 4)
. • The type and sequence of the amino acids joined together will determine the kind of protein formed.
• Each group of three bases stands for one amino acid.• A gene is a sequence of triplets of the four bases, which codes for one protein molecule.• Insulin is a small protein with only 51 amino acids and so 153 (i.e. 3 × 51) bases in the DNA molecule. Most proteins are much larger than this and most genes contain a thousand or more bases.
The chemical reactions
The chemical reactions that take place in a cell determine the type of cell it is and what its functions are. These chemical reactions are, in turn, controlled by enzymes. Enzymes are proteins. So, by determining which proteins (particularly enzymes) are produced in a cell, the genetic code of DNA also determines the cell’s structure and function. In this way, the genes also determine the structure and function of the whole organism. Other proteins coded for in DNA include antibodies and the receptors for neurotransmitters (see details of synapses in Chapter 14).
gene expression
Body cells do not all have the same requirements for proteins. For example, the function of some cells in the stomach is to make the protein pepsin (see ‘Chemical digestion’ in Chapter 7). Bone marrow cells make the protein haemoglobin, but do not need digestive enzymes. Specialised cells all contain the same genes in their nuclei, but only the genes needed to code for specific proteins are switched on (expressed). This enables the cell to make only the proteins it needs to fulfil its function.
diploid nucleus and haploid nucleus
The definitions of diploid nucleus and haploid nucleus are given at the start of this chapter. In each diploid cell (nearly all body, or somatic, cells) there is a pair of each type of chromosome (see Figure 17.1). In a human diploid cell, there are 23 pairs. Sex cells (sperm and ova) are haploid, containing only 23 chromosomes. The 23 chromosomes comprise one from each pair. Each chromosome is made up of a large number of genes coding for the formation of different proteins that give us our characteristics.
The definition of mitosis is
given at the start of this chapter. Mitosis is a form of cell division used for making new cells to enable growth or the replacement of old or damaged cells. Asexual reproduction involves mitosis.
The definition of meiosis
is given at the start of this chapter. Sex cells (gametes) are formed in the gonads (ovaries and testes) by meiosis. When ova are formed in a woman, all the ova will carry an X chromosome. When sperm are formed in a man, half the sperm will carry an X chromosome; half will carry a Y chromosome (see Figure 17.3).
formation of sex cells by meiosis





