Beyond Mendel — EST 2 Biology Revision Notes & Topical Questions

An organism's genotype is the set of alleles it inherits for one or more genes. Homozygous dominant and homozygous recessive parents form the parental (P) generation. Their offspring are the F1 generation and the offspring of the F1 are the F2 generation. A Punnett grid predicts the probability of a given genotype or phenotype.

Monohybrid crosses follow how the alleles of one gene pass between generations. Dihybrid crosses follow two genes.

In another cross a result of 3 BbEE : 3 bbEE : 1 Bbee : 1 bbee is the predicted ratio of genotypes in the offspring; the phenotypes are then found by applying dominance to each gene.

A pedigree maps out patterns of inheritance and lets us predict the inheritance of genes, even for linked genes. Reading a pedigree:

Worked example, albinism (a condition affecting the pigment melanin, giving lighter hair, skin and eyes).

Some patterns of inheritance do not follow Mendel's laws: the observed phenotypic ratios among the offspring differ significantly from the predicted ratios. These patterns can be identified by quantitative analysis, comparing observed results with expected ratios.

One example is gene linkage, where some genes are not inherited independently because they lie close together on the same chromosome. Linked genes produce many more parental-type offspring than the expected Mendelian ratio predicts.

DNA controls cell function by controlling the production of proteins. The sequence of bases in a gene determines the amino acid sequence of a protein, and the proteins (enzymes, structural proteins and others) determine what a cell can do. Control of gene expression decides which genes are used in which cells.

Meiosis is a type of nuclear division that produces cells that are genetically different, whereas other divisions produce cells needed for repair, which replaces damaged or dead cells.

A gene can be copied and used in host organisms. In gene cloning:

Transformation is the uptake of foreign DNA by a cell, for example a bacterium taking up a plasmid.

During prophase I of meiosis, homologous chromosomes pair and non-sister chromatids exchange corresponding DNA segments at chiasmata. This crossing over creates recombinant chromosomes. Genes close together on one chromosome are linked and tend to be inherited together; the greater the distance between loci, the higher the chance of crossing over between them.

Phenotypic plasticity occurs when individuals with the same genotype show different phenotypes in different environments. The genotype stays fixed over the organism's life, but how it is expressed can vary because internal or external conditions influence gene expression.

Environmental stressors directly influence protective phenotypic traits, showing how environment and genes interact.

Inheritance is the transmission of genetic information from generation to generation. Chromosomes are found in the nucleus. A useful zoom model starts with the whole organism, zooms in to the chromosomes in the nucleus, then to individual genes and then to the bases of DNA.

Sex is decided by a whole chromosome pair, not by one or a few genes.

The DNA base sequence determines the amino acid sequence in a protein. DNA is too large to leave through a nuclear pore, so protein synthesis occurs in two stages.

Mendel-type genotype ratios predict only the alleles inherited. They do not predict how the phenotype will appear when the environment changes expression. Therefore exam answers on phenotype must consider both genotype and environment. When a phenotype differs between organisms with the same genotype, the cause is environmental influence on gene expression rather than a new allele.

Other non-Mendelian patterns include linked genes and DNA in mitochondria and chloroplasts, which are not inherited in the usual nuclear way.

During meiosis, homologous chromosomes (one from each parent) pair up in prophase I. Non-sister chromatids lie side by side and may break and rejoin, swapping corresponding segments of DNA. The points where the chromatids cross are called chiasmata (singular: chiasma).

The result is a recombinant chromosome: one that carries a new mixture of alleles that did not exist together on either parental chromosome. Crossing-over is therefore a source of genetic variation, in addition to the random assortment of chromosome pairs.

These cells become gametes, for example sperm, egg cells, pollen grains and ova.

Mendel's rule of independent assortment works when genes are on different chromosomes. Genes that lie on the same chromosome are linked: they travel together into the same gamete and tend to be inherited as a block.

Because the chance of crossing-over rises with distance, the proportion of recombinant offspring can be used as a rough measure of how far apart two genes are on a chromosome.

Gene cloning makes many identical copies of a gene by placing it in a host cell that divides repeatedly.

In Mendelian genetics each characteristic is controlled by a gene with two alleles, one inherited from each parent. Alleles segregate during gamete formation, so each gamete carries only one allele of each gene. At fertilisation, alleles are combined at random.

A Punnett square lists every possible gamete from each parent along the edges and every possible offspring genotype in the boxes. The ratio of boxes gives the predicted ratio of genotypes and phenotypes in the offspring.

The ABO blood group is controlled by one gene with three alleles: I^A, I^B and I^O. I^A and I^B are codominant, meaning both are expressed in a heterozygote. I^O is recessive to both.

An A allele with an O allele gives group A, a B allele with an O allele gives group B, and two O alleles give group O.

Take a parent of group A (I^AI^O) and a parent of group B (I^BI^O). Their gametes are I^A or I^O, and I^B or I^O.

These are probabilities for each child. A family with four children need not have one of each group, because every conception is an independent event.

Notice also that if parents of group A and B have a group O child, both parents must be heterozygous, because the child must have inherited an I^O allele from each.

Some genes, such as the gene for red-green colour vision, sit on the X chromosome. A male has only one X, so a single recessive allele on it is expressed. A female needs two recessive alleles to be affected, so X-linked recessive traits are more common in males. A female with one recessive allele is a carrier.

In a cross between a normal male and a carrier female, each child has a 25% chance of being each of: colour-blind male, carrier female, normal female, normal male.

In incomplete dominance, the heterozygote shows an intermediate phenotype. For example, red (RR) × white (WW) snapdragons give pink (RW) offspring; crossing two pink plants gives 1 red : 2 pink : 1 white, so the genotype and phenotype ratios are the same.

In codominance, both alleles are fully expressed in the heterozygote. The human ABO blood group shows this: alleles I^A and I^B are codominant, so I^AI^B gives blood group AB. The allele i is recessive.

Three alleles exist in the population (multiple alleles), though each person has only two. Polygenic inheritance occurs when many genes affect one trait (height, skin colour), giving continuous variation with a bell-shaped distribution, and the environment also contributes.

Genes on the X chromosome show sex-linked inheritance. Males (XY) have only one X, so a recessive allele on it is expressed (red-green colour blindness, haemophilia). Females (XX) need two copies and may be carriers.

Example

• a carrier mother (X^HX^h) and unaffected father (X^HY): 50% of sons have haemophilia, no daughters are affected but 50% are carriers.
• An affected father cannot pass the allele to his sons, as sons receive his Y chromosome.

Linked genes lie close together on the same chromosome and tend to be inherited together, so dihybrid ratios differ from 9 : 3 : 3 : 1. Crossing over during meiosis produces some recombinants; the recombination frequency reflects the distance between genes and is used to map chromosomes.

Epistasis is when one gene masks or modifies the effect of another.

A sound strategy for problems is to look at the ratio in the offspring.

Chi-squared analysis tests whether the observed values differ from expected ones by chance: χ² = Σ(O − E)²/E, with degrees of freedom = classes − 1. If the value is below the critical value at p = 0.05, the difference is not significant, and the hypothesis is accepted.