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
mitosis explanation
Before the process starts, all the chromosomes are duplicated exactly. During mitosis, the copies of the chromosomes separate and form two nuclei with the same number of chromosomes as the parent nucleus cell (the diploid number of chromosomes is maintained). At the end of a mitotic cell division, the number of cells is doubled and the daughter cells produced are genetically identical to the parent.
stem cells
Stem cells are unspecialised cells in the body that have retained their power of division by mitosis. The daughter cells produced can become specialised for specific functions. Examples include the basal cells of the skin, which keep dividing to make new skin cells, and cells in the red bone marrow, which constantly divide to produce the whole range of blood cells. Cells taken from early embryos (embryonic stem cells) can be induced to develop into almost any kind of cell. Scientists have learned to re-programme skin cells so that they develop into other types of cell, such as nerve cells.
Meiosis explanation
The definition of meiosis is given at the start of this chapter. Note that there is a more detailed version needed for the Extended paper.
Meiosis is called a reduction division because it involves halving the normal chromosome number – the pairs of chromosomes are separated. The gametes (sex cells) produced are haploid, but they are formed from diploid cells. At the end of the process, the cells produced are not all identical – meiosis results in genetic variation.
Both the maternal and paternal chromosomes contain new combinations of genetic material.
monohybrid inheritence
Monohybrid inheritance involves the study of how a single gene is passed on from parents to offspring. It is probably easiest to predict the outcome of a monohybrid cross using a Punnett square (see Figure 17.4). However, if you have been taught the traditional way of displaying the cross (as shown in Figure 17.2), there is nothing wrong with using that method.
All the genetic crosses shown will involve examples using pea plants, which can be tall (T) or dwarf (t) – tall is dominant to dwarf.
If two identical homozygous
If two identical homozygous individuals are bred together, the product of the cross will be pure-breeding. However, if one parent is pure-breeding dominant and the other parent is pure-breeding dwarf, there will be a different outcome, as shown in the first example on the next page. A cross between a pure-breeding tall pea plant and a pure-breeding dwarf pea plant: As tall is dominant to dwarf, and both plants are pure-breeding, their genotypes must be TT and tt.
homozygous breeding
hetrozygous cross
Pedigree
The term pedigree often refers to the pure-breeding nature of animals, but is also used to describe human inheritance. Pedigree diagrams are similar to family trees and can be used to demonstrate how genetic diseases can be inherited. They can include symbols to indicate whether individuals are male or female and what their genotype is for a particular genetic characteristic. One genetic disease is called cystic fibrosis. Cystic fibrosis sufferers tend to have a much shorter lifespan than normal and suffer from respiratory, digestive and reproductive problems. A sufferer of cystic fibrosis has two recessive alleles (cc). A carrier of the disease has one normal allele and one recessive allele (Cc). A healthy non-carrier has two normal alleles (CC). • A man who is not a carrier (CC) who has children with a woman who is not a carrier (CC) will produce 100% children who are not carriers (all CC). • If one parent is a carrier for cystic fibrosis (Cc) and the other parent is not a carrier (CC), 50% of their children are likely to be carriers (Cc) and 50% will be not be carriers (CC). • However, if both parents are carriers, then the likely ratio of offspring of non-carriers/ carriers/ cystic fibrosis sufferers (CC:Cc:cc) is 1: 2: 1. So, there is a 1 in 4 chance of a child born to these parents having cystic fibrosis. The pedigree diagram (Figure 17.7) shows the inheritance of cystic fibrosis in a family.
Using a test-cross (back-cross)
A test-cross can be used to identify an unknown genotype. For example, a black mouse could have either the BB or the Bb genotype. One way to find out which it has is to cross the black mouse with a known homozygous recessive mouse (bb, having the phenotype of brown fur). The bb mouse will produce gametes with only the recessive b allele. A black homozygote (BB) will produce only B gametes. BB × bb will produce 100% black individuals (all Bb). Bb × bb will produce, on average, 50% black individuals (Bb) and 50% brown individuals (bb). This outcome identifies a parent that is not pure-breeding.
codominance
This term describes a pair of alleles, neither of which is dominant over the other. This means that both can have an effect on the phenotype when they are present together in the genotype. The result is that there can be three different phenotypes. When writing the genotypes of co-dominant alleles, the common convention is to use a capital letter to represent the gene involved, and a small raised (superscript) letter for each phenotype.
Inheritance of A, B, AB and O blood groups These blood groups
These blood groups give an example of co-dominance. Instead of two alleles being present, in this case there are three: IA, IB and IO. Combinations of these can result in four different phenotypes: A, B, AB and O. The alleles are responsible for producing antigens that respond to foreign antibodies (this can result in blood clotting in blood transfusions, and rejection of organs after transplant operations). However, while IA and IB are co-dominant, IO is dominated by both the other alleles. This means, for example, that a person with blood group A could have the genotype IAIA or IAIO. This has implications when having children because, if both parents carry the IO allele, a child could be born with the genotype IOIO (blood group O), even though neither of the parents have this phenotype.
sex linkage
The definition of a sex-linked characteristic is given at the start of this chapter. The sex chromosomes (X and Y) carry genes that control sexual development. In addition, they carry genes that control other characteristics. These tend to be on the X chromosome, which has longer arms to the chromatids. Even if the allele is recessive, because there is no corresponding allele on the Y chromosome, it is bound to be expressed in a male (XY). There is less chance of a recessive allele being expressed in a female (XX) because the other X chromosome may carry the dominant form of the allele. One example of this is a form of colour blindness (Figure 17.9). In the following case, themother is a carrier of colour blindness (XCXc). This means that she shows no symptoms of colour blindness, but the recessive allele causing colour blindness is present on one of her X chromosomes. The father has normal colour vision (XCY). If the gene responsible for a particular condition is present on only the Y chromosome, only males can suffer from the condition because females do not possess the Y chromosome.
DNA molecules remain in the nucleus, but
the proteins that they carry the codes for are needed elsewhere in the cell.
A molecule called messenger RNA (mRNA
is used to transfer the information from the nucleus.
An RNA molecule is much smaller than a DNA molecule
and is made up of only one strand.
To pass on the protein code, the double helix of DNA
unwinds to expose the chain of bases.
One strand acts as template. An mRNA molecule is formed
along part of this strand, made up of a chain of nucleotides with complementary bases to a section of the DNA strand.
The mRNA molecule carrying the protein code then moves out of the nucleus
into the cytoplasm, where it passes through a ribosome.
The mRNA molecule instructs the ribosome to put together a chain of amino acids
n a specific sequence, thus making a protein. Other mRNA molecules will carry codes for different proteins.
