Chromosomes and inheritance — EST 2 Biology Revision Notes & Topical Questions

All the cells in the body, except the gametes, are produced by mitosis of the zygote. Mitosis makes new cells that are genetically identical to the parent cell.

Meiosis is a type of nuclear division that gives rise to cells that are genetically different. It is involved in the production of gametes (sex cells), such as sperm and egg cells or pollen grains and ova.

The number of chromosomes must be halved when gametes are formed. Otherwise the zygote, formed when two gametes join at fertilisation, would have double the number of chromosomes.

Inheritance is the transmission of genetic information from one generation to the next. A gene is a short length of DNA on a chromosome that codes for a particular characteristic.

Alleles are versions, or forms, of a gene. Because chromosomes exist in matching pairs, an individual has two copies of each gene and so two alleles. The two can carry the same instructions or different instructions.

It is not always possible to find the genotype just by looking at the phenotype. A phenotype linked to a dominant allele appears in both the dominant homozygous and the heterozygous genotype. Two individuals who are both homozygous for the same characteristic produce offspring with exactly the same genotype and phenotype as themselves.

The height of pea plants is controlled by one gene with two alleles. The tall allele is dominant and written T; the short allele is recessive and written t. Tall plants are TT or Tt, and short plants are tt.

Step 1 is to write the gametes of each parent, and step 2 is to determine the probability from the Punnett square.

Mitosis and meiosis both begin with chromosome copying, but their outcomes are very different. A comparison table is the clearest way to remember the contrasts that exam questions are built on.

Notice that meiosis, unlike mitosis, includes recombination, in which paired chromosomes exchange segments before being pulled apart. This is why gametes from the same parent are not all identical, and why brothers and sisters can look different even though they share the same parents.

Show the results of crossing a heterozygous tall plant with a short plant. The heterozygous parent is Tt and the short parent is tt. Gametes: T or t from the first parent, and only t from the second.

Offspring genotypes are Tt, Tt, tt, tt. Phenotypes are 2 tall : 2 short, so the probability of a short offspring is 50% and the probability of a tall offspring is 50%.

A Punnett square answers all three.

A good answer shows the parent gametes, the offspring genotypes, and an indication of which offspring have the characteristic asked for, and then states the probability.

If you are asked to use your own letters, pick a letter whose capital and lower-case forms look clearly different, so the examiner is not left in doubt about which allele is dominant. For example, C and c are not very different when written by hand, whereas A and a are very different.

Always use the capital letter for the dominant allele and the lower-case for the recessive allele, and state a key such as A = tall, a = short.

Family pedigree diagrams trace the pattern of inheritance of a characteristic, usually a disease, through the generations of a family. Squares usually represent males and circles females; shaded symbols show affected individuals.

A pedigree can show, for example, that:

To find genotypes, work from individuals you can be sure about. If two unaffected parents have an affected child, the allele is recessive and both parents are carriers. An affected child must therefore have received one recessive allele from each parent.

Codominance occurs when both alleles in a heterozygous organism contribute to the phenotype.

The zoom model helps you picture where you are in a cell.

Chromosomes are located in the nucleus of cells. Inheritance is the transmission of genetic information from generation to generation, and it is carried in the DNA of the chromosomes.

All humans have 23 different chromosomes in each cell, and most body cells hold two copies of each, making 46. This is the diploid number. Haploid means half the normal number of chromosomes, so human gametes have 23.

In an exam you may be given information about a different species with a different chromosome number. Read the question carefully: use the number given, not 46.

Before a cell divides by mitosis, the chromosomes double (each copies itself), so that when the cell splits in two, each new cell still has two copies of each chromosome and is still diploid. Mitosis is described in steps:

Sex is determined by an entire chromosome pair, unlike most other characteristics, which are determined by just one gene or a number of genes. Females are XX and males are XY.

Because only a father can pass on a Y chromosome, he is responsible for determining the sex of the child.

The inheritance of sex is shown with a Punnett square in which the X and Y chromosomes replace the alleles. The probability of a boy or a girl is 50% for each pregnancy.

The DNA base sequence determines the amino acid sequence in a protein. Expression of a gene means whether that gene is transcribed and translated in a particular cell or not.

In this way DNA controls cell function by controlling the production of proteins.

Meiosis is the cell division that produces haploid gametes such as sperm cells, egg cells, pollen grains and ova.

Recombination means the daughter cells differ from each other and from the parent cell.

Inheritance is the transmission of genetic information from one generation to the next. A gene is a short length of DNA on a chromosome that codes for a particular characteristic. Alleles are versions of a gene.

Chromosomes exist in matching pairs, so an individual has two copies of each gene and therefore two alleles of each gene. An individual can only inherit two alleles for each gene; they can be the same or different, carrying the same instructions or different instructions.

If you choose your own letters, avoid pairs such as C and c that look alike when handwritten; A and a are very different.

Pedigree diagrams trace a characteristic, usually a disease, through generations. A pedigree may show that both males and females are affected, that every generation has affected individuals, and that one family group has no affected parents or children.

Meiosis explains why the Punnett square works.

Recombination adds further variation. Because paired chromosomes exchange DNA, the alleles in a gamete may be a new mixture compared with either of the parent's own chromosomes. Fertilisation then adds more variation, because any gamete from one parent can fuse with any gamete from the other.

These three sources explain why offspring of the same parents differ from each other and why the observed numbers in a real cross are close to, but rarely exactly, the predicted ratio.

A family pedigree diagram traces the pattern of inheritance of a specific characteristic, usually a disease, through generations of a family. The pedigree in this topic shows three features:

Look first for unaffected parents with an affected child, which proves a recessive allele, then trace the carriers back through the generations.

Codominance occurs when both alleles in a heterozygous organism contribute to the phenotype. Inheritance of blood group is an example. There are three alleles of the gene: I^A, I^B and I^O.

In a worked Punnett square, a parent with blood group A and a parent with group B have a child with group O.

Colour blindness is an X-linked condition. The allele is carried on the X chromosome and is recessive. A normal male is X^NY; a carrier female is X^NXⁿ.

In this cross there is a 25% chance of producing a colour-blind male, 25% chance of a carrier female, 25% chance of a normal female and 25% chance of a normal male. Males are affected more often because they have only one X.

Chromosomes are located in the nucleus of cells.

DNA cannot travel out of the nucleus to the ribosomes, because it is far too big to pass through a nuclear pore. Instead:

Expression of a gene means whether it is transcribed and translated in a particular cell.

All humans have 23 different chromosomes, in pairs, giving 46 in each body cell. Haploid means half the normal number. In exams, check the number given for other species.

New cells made by mitosis start by copying every chromosome, so each new cell still has two copies of each chromosome and is diploid.

Meiosis gives genetically different cells and makes gametes. The number must be halved, otherwise the zygote would have double. The first division separates pairs after recombination, the second division separates the copies, and four haploid cells are formed.

Inheritance is the transmission of genetic information from one generation to the next. A gene is a short length of DNA found on a chromosome that codes for a particular characteristic. Alleles are versions of a gene. Since chromosomes exist in matching pairs, an individual has two copies of each gene and therefore two alleles.

It is not always possible to deduce genotype from phenotype. If two individuals homozygous for the same characteristic are bred, offspring have exactly the same genotype and phenotype as the parents.

To work out ratios, percentage chances or phenotypes, follow these steps:

If you choose your own letters, pick a pair with an obvious difference between capital and lower case: C and c are not very different when handwritten, whereas A and a are.

Pedigrees trace a characteristic, usually a disease, through generations. In the example, both males and females are affected, every generation has affected individuals, and one family group has no affected parents or children. Use them with the Punnett method to decide whether the allele is dominant or recessive and which individuals must be heterozygous.

Codominance occurs when both alleles in a heterozygous organism contribute to the phenotype. Inheritance of blood group is an example of codominance. There are three alleles of the gene that governs the inheritance of blood group: I^A, I^B and I^O.

In the blood-group Punnett square, the parent with group A must have the genotype I^AI^O and the parent with group B must have the genotype I^BI^O.

Colour blindness is an X-linked recessive condition. The results of a cross between a normal male (X^NY) and a female carrier (X^NXⁿ) are as follows.

A son always gets his Y from the father and his X from the mother, which is why a son of a carrier mother has a 50% chance of being colour-blind, while no daughter of a normal father can be colour-blind.

Write the sex chromosomes in the Punnett square instead of single letters, put the allele as a superscript on the X, and remember that Y carries no allele for the gene. A female needs two recessive alleles to be affected, but a male needs only one.

Codominance and sex-linkage are different ideas that are easily confused, so compare them directly.

In an autosomal codominant cross, the sex of the child does not matter. In an X-linked cross, the sex of the child is part of the answer, so the table must show both sexes and the question may ask for the chance of a particular sex with a particular phenotype.

As a final check, use these rules.