Chapter 16: Inherited Change

Homologous Chromosomes

Homologous Chromosomes are pairs of chromosomes in a diploid cell that have the same structure and the same genes, but not necessarily the same varieties of those genes.

Diploid is a cell or organism that has paired chromosomes, one from each parent. In humans, cells other than human sex cells, are diploid and have 23 pairs of chromosomes. Human sex cells (egg and sperm cells) contain a single set of chromosomes and are known as haploid.

Homologous chromosomes are made up of chromosome pairs of approximately the same length, centromere position, and staining pattern, for genes with the same corresponding loci. One homologous chromosome is inherited from the organism’s mother; the other is inherited from the organism’s father.

Homologous chromosomes are important in the processes of meiosis and mitosis. They allow for the recombination and random segregation of genetic material from the mother and father into new cells.

Meiosis

Meiosis is a form of eukaryotic cell division. Meiosis gives rise to four unique daughter cells, each of which has half the number of chromosomes as the parent cell because meiosis creates cells that are destined to become gametes (or reproductive cells).

 

The first round of nuclear division that occurs during the formation of gametes is called meiosis I. It is also known as the reduction division because it results in cells that have half the number of chromosomes as the parent cell. Meiosis I consist of four phases: prophase I, metaphase I, anaphase I, and telophase I.

Prophase I

  • DNA condenses and becomes visible as chromosomes
  • DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere
  • The chromosomes are arranged side by side in homologous pairs
    • A pair of homologous chromosomes is called a bivalent
  • As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
  • In this stage centrioles migrate to opposite poles and the spindle is formed
  • The nuclear envelope breaks down and the nucleolus disintegrates

Metaphase I

  • The bivalents line up along the equator of the spindle, with the spindle fibers attached to the centromeres

Anaphase I

  • The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
  • The centromeres do not divide

Telophase I

  • The chromosomes arrive at opposite poles
  • Spindle fibres start to break down
  • Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
  • Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Cytokinesis

  • This is when the division of the cytoplasm occurs
  • Cell organelles also get distributed between the two developing cells
  • In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
  • In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell

The second division of meiosis is almost identical to the stages of mitosis

  • Prophase II
    • The nuclear envelope breaks down and chromosomes condense
    • A spindle forms at a right angle to the old one
  • Metaphase II
    • Chromosomes line up in a single file along the equator of the spindle
  • Anaphase II
    • Centromeres divide and individual chromatids are pulled to opposite poles
    • This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell
  • Telophase II
    • Nuclear membranes form around each group of chromosomes

Meiosis produces genetically different cells; genetic variation is achieved through:

  • Crossing over chromatids where pairs of chromosomes line up and exchange some of their genetic material
  • Independent assortment of chromosomes- there are various combinations of chromosome arrangement

Crossing over

  • Crossing over is the process by which non-sister chromatids exchange alleles
  • Process:
    • During meiosis I homologous chromosomes pair up and are in very close proximity to each other
    • The non-sister chromatids can cross over and get entangled
    • These crossing points are called chiasmata
    • The entanglement places stress on the DNA molecules
    • As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
  • This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
  • There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
  • Crossing over is more likely to occur further down the chromosome away from the centromere.

Genotype & phenotype

  • The chromosomes of eukaryotic cells occur in homologous pairs (there are two copies of each chromosome)
  • As a result cells have two copies of every gene
  • As there are two copies of a gene present in an individual that means there can be different allele combinations within an individual
  • The genotype of an organism refers to the alleles of a gene possessed by that individual. The different alleles can be represented by letters
  • When the two allele copies are identical in an individual they are said to be homozygous
  • When the two allele copies are different in an individual they are said to be heterozygous
  • The genotype of an individual affects their phenotype
  • A phenotype is the observable characteristics of an organism

Dominance and Codominance

Dominance, in genetics, greater influence by one of a pair of genes (alleles) that affect the same inherited character.

Codominance is a relationship between two versions of a gene. Individuals receive one version of a gene, called an allele, from each parent. If the alleles are different, the dominant allele usually will be expressed, while the effect of the other allele, called recessive, is masked.

Monohybrid and Dihybrid cross

Monohybrid cross:  A monohybrid cross can be defined as a genetic mix between two individuals who have homozygous genotypes or genotypes which have completely dominant or recessive alleles.  This results in opposite phenotypes for a specific genetic trait.

Monohybrid cross experiments are carried out by geneticists to study how the offspring of homozygous individuals express the heterozygous genotypes they inherit from their parents. The cross also signifies a genetic mix between two individuals who have heterozygous genotypes which confirm the dominance of an allele. 

Example: A well-noted example of a monohybrid cross is Mendel’s experiments on pea plants which helped him narrate the concept of genes and the law of segregation and independent assortment. 

  • One of the genes for the coat colour of horses has the following two alleles:
    • B, a dominant allele produces a black coat when present
    • b, a recessive allele produces a chestnut coat when present in a homozygous individual
  • In this example a heterozygous male is crossed with heterozygous female

Parental phenotype:   black coat x black coat

Parental genotype:     Bb                   Bb

Parental gametes:      B or b              B or b

Monohybrid punnett square with heterozygotes table

  • Predicted ratio of phenotypes in offspring – 3 black coat : 1 chestnut coat
  • Predicted ratio of genotypes in offspring – 1 BB : 2 Bb : 1 bb
  • Monohybrid punnett square with sex-linkage table
  • Predicted ratio of phenotypes in offspring – 1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting
  • Predicted ratio of genotypes in offspring: 1 XFXF: 1 XFXf : 1 XFY : 1 XfY

Di-hybrid Cross: A dihybrid cross is another experiment in genetics that is carried out to follow the behavior of the phenotypes of two genes through the mating of individuals carrying multiple alleles at those gene loci.  Simply put, it’s a cross between two observed states, where the homozygous dominant traits are crossed with homozygous recessive traits and in the first generation; all are heterozygous where the dominant phenotypic traits are observed. However, all the offspring will be carriers of the recessive traits. The name di-hybrid indicates that there are two traits involved and each trait has two different alleles. 

Example: Mendel’s experiments on the pea plants can be taken as an example here as well. The traits that were taken were yellow and round seeds and green and wrinkled seeds. Two different phenotypes were crossed which gave rise to four different phenotypes in the F2 generation with a ratio of 9:3:3:1 and the genotypic ratio was 1:2:2:4:1:2:1:2:1. The observations shed light on the mode of inheritance in an organism. Based on the results, Mendel also created the laws of independent assortment.

Between Monohybrid and Dihybrid

Monohybrid

Dihybrid

Means

Mono refers to single and hybrid means mixed breed

Di refers to two or double and hybrid means breed

Cross

Monohybrid cross is used to study the inheritance of a single pair of alleles

Dihybrid cross is used to study the inheritance of 2 different alleles

Used to study

the dominance of genes

Offspring assortment

Genotype ratio

1:2:1

1:2:1:2:4:2:1:2:1

Phenotype ratio

3:1

9:3:3:1

Ratio cross test

-1:1

-1:1:1:1

Mutation

A mutation is a change in a DNA sequence. A Mutation occurs when a DNA gene is damaged or changed in such a way as to alter the genetic message carried by that gene. A Mutagen is an agent of substance that can bring about a permanent alteration to the physical composition of a DNA gene such that the genetic message is changed. Mutations can result from DNA copying mistakes made during cell division, exposure to ionizing radiation, exposure to chemicals called mutagens, or infection by viruses.

Types of mutation are:

  • Germline mutations occur in gametes. Somatic mutations occur in other body cells.
  • Chromosomal alterations are mutations that change chromosome structure.
  • Point mutations change a single nucleotide.
  • Frameshift mutations are additions or deletions of nucleotides that cause a shift in the reading frame.

Albinism

Albinism is a condition that affects a person’s melanin production in the skin and leads to them having white hair, light eyes and pale skin. The TYR gene is responsible for the production of tyrosinase. This is the enzyme that controls melanin production. If a mutation in the TYR gene occurs, tyrosinase production is hindered resulting in the person developing albinism.

Sickle cell Anemia

Sickle cell disease is caused by a mutation in the hemoglobin-Beta gene found on chromosome 11. Sickle-cell anemia is the result of a change in a single nucleotide, and it represents just one class of mutations called point mutations. Changes in the DNA sequence can also occur at the level of the chromosome, in which large segments of chromosomes are altered.

Hemophilia

Hemophilia is usually an inherited bleeding disorder in which the blood does not clot properly. Hemophilia is caused by a mutation or change, in one of the genes, that provides instructions for making the clotting factor proteins needed to form a blood clot.

Hemophilia is inherited in an X-linked recessive pattern. A condition is considered X-linked when gene mutation that causes it is located on the X chromosome, one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is enough to cause the condition.

Chi-squared Test

A chi-squared test (also chi-square or χ2 test) is a statistical hypothesis test that is valid to perform when the test statistic is chi-squared distributed under the null hypothesis, specifically Pearson’s chi-squared test and variants thereof. Pearson’s chi-squared test is used to determine whether there is a statistically significant difference between the expected frequencies and the observed frequencies in one or more categories of a contingency table. Chi-squared tests often refers to tests for which the distribution of the test statistic approaches the χ2 distribution asymptotically, meaning that the sampling distribution (if the null hypothesis is true) of the test statistic approximates a chi-squared distribution more and more closely as sample sizes increase

An experiment was carried out investigating the inheritance of two genes in rabbits; one for coat colour and one for ear length. A dihybrid cross revealed the expected ratio of phenotypes to be 9 : 3 : 3 : 1. Several rabbits with the heterozygous genotype were bred together and the phenotypes of all the offspring were recorded. The ratio of the offspring was not exactly what was predicted. In order to determine whether this was due to chance or for some other reason, the chi-squared test was used.

Chi-squared worked example table

The expected ratio is calculated by multiplying the total number of organisms ( 128 rabbits) with each expected ratio:

  • In order to understand what this chi-squared value of 0.56 says about the data, a table relating chi-squared values to probability is needed
  • The chi-squared table displays the probabilities that the differences between expected and observed are due to chance
  • The degrees of freedom can be worked out from the results. It is calculated by subtracting one from the number of classes
    • In this example there are four phenotypes which means four classes, 4 – 1 = 3
    • This means that the values in the third row are important for comparison
  • For this experiment, there is a critical probability of 0.05
    • This means that 7.82 is the value used for comparison
  • The chi-squared value from the results (0.56) is much smaller than 7.82
  • 0.56 would be located somewhere to the left-hand side of the table, representing a probability much greater than 0.1

This means that there is no significant difference between the expected and observed results, any differences that do occur are due to chance.