GENETICS

1. GENETICS 4.1

2. 4.1.1 State that eukaryote chromosomes are made of DNA and proteins • Chromosomes are bundles of long strands of DNA. • Chromosomes always come in pairs except in sex cells or gametes. • Humans have 46 chromosomes in 23 pairs. • The DNA in eukaryotes is associated with proteins which help to keep the DNA organized. • Prokaryotes have only one chromosome, and the DNA is not associated with proteins.

3. Chromosome Each vertical rod of chromosome is called a chromatid. The area where the chromatids are joined together is called the centromere.

4. Homologous Chromosome • Homologous chromosomes (also called homologs or homologues) are chromosome pairs of approximately the same length, centromere position, with genes for the same characteristics at corresponding loci. • One homologous chromosome is inherited from the organism's mother; the other from the organism's father.They are usually not identical. • Each chromosome in the pair contains genes for the same biological features, such as eye color, at the same locations (loci) on the chromosome. However, each can contain either the same allele (e.g., both alleles for blue eyes) or different alleles (e.g., one allele for blue eyes and one allele for brown eyes) for each feature.

5. Homologous chromosomes • Humans have 22 pairs of homologous non-sex chromosomes (called autosomes), and one pair of sex chromosomes, making a total of 46 chromosomes in a genetically normal human. Each member of a pair is inherited from one of the two parents. In addition to the 22 pairs of homologous autosomes, female humans have a homologous pair of sex chromosomes (two Xs), while males have an X and a Y chromosome.

6. Human Karyotype • Chromosomes arranged as homologous pairs in descending order of size

7. Genes • A gene is a heritable factor that controls a specific characteristic. • The estimated 30 000 genes which you possess are organized into chromosomes. • Gene can be defined as a unit of inheritance. • It is also a specific length of DNA, which codes for proteins. • The position of a gene is called a locus. • Each gene has two or more forms called alleles. • The complete set of an organism’s base sequence is called its genome.

8. Allele • An allele is any of the alternative forms of a gene that may occur at a specific locus. • Body cells have two alleles for each gene, one on each of the homologous chromosome on which the locus for that gene is found. Each parent gives one allele. • The two alleles may be the same, or they may may be different. • The term homozygous describes two of he same alleles at a specific locus. For example, both might code for white flower. • The term heterozygous describes two different alleles at a specific locus. Thus, one might code for white flowers, the other for purple flowers.

9. Alleles • In some people, earlobes are attached and in others, they are not. The gene for this trait comes in two possible forms: one allele for attached earlobes and one allele for non-attached earlobes. • The genes which determine eye color have more than one form. Such variations of a gene are called alleles. • An allele is one specific form of a gene differing from other alleles by one or a few bases. • Alleles of the same gene occupy a corresponding (locus) on each chromosome of a pair.

10. -- • If you have a gene that influences blood cholesterol levels on chromosome 8, you will have one copy from your mother and one copy from your father. It is possible that one of these copies is associated with high cholesterol levels, while the other is associated with low cholesterol levels. Then we call this as heterozygous allele.

11. Replication of Chromosomes • If a nucleus is going to divide by mitosis or meiosis, all DNA in the nucleus is replicated. When mitosis or meiosis begins, each chromosome is visible as a double structure. The two parts are called chromatids and are connected by a centromere. The position of the centromere varies.

12. Mutations • Genes are almost always passed from parent to offspring without being changed. Occasionally genes do change and this is called gene mutation. • Gene mutation is a change to the base sequence of a gene. The smallest possible change is when one base in a gene is replaced by another base. This type of mutation is called base substitution. • Many gene mutations cause a genetic disease. One example is sickle cell anemia

14. Base substitution mutation • The kind of mutation which causes sickle cell anemia is called a base substitution mutation. • The symptoms of sickle cell anaemia are weakness, fatigue and shortness of breath. • Oxygen cannot be carried efficiently and the hemoglobin tends to crystallize within the red blood cells, causing them to be less flexible. The affected blood cells can get stuck in capillaries so blood flow can be slowed or blocked. • People affected by sickle cell anemia have a risk of passing the mutated gene to their offspring.

15. Sickle- cell anemia • The frequency of sickle cell is correlated with the prevalence of malaria in many parts of the world. • There has clearly been natural selection in favour of the sickle-cell allele in malarial areas, despite it causing severe anemia in the homozygous condition. Natural selection has led to particular frequencies of the sickle-cell and the normal hemoglobin alleles, to balance the twin risks of anemia and malaria.

16. Typical red blood cells and one sickle-shaped one