Chapter 22: Heredity

Genetics is the study of Heredity and genes.

Heredity is the study of of what causes resemblance and differences b/w organism.

• Homozygous

  • An organism containing two identical alleles of a particular gene.

  • If the mother and father pass down the same allele for a particular trait, e.g. they both pass down the allele for blue eyes, the offspring will have two identical alleles for this trait, which is referred to as homozygous.

• Chromosome

  • Is a thread like structure found in nucleus on which the genetic material is organized.

  • Carries genetic information in the form of genes.

• Gene

  • A length of DNA that codes for a particular protein.

• Allele

  • Alternative form of gene.

  • Occupy the same relative positions on a pair of homologous chromosomes.

• Genotype

  • Is the genetic characteristic of an organism.

  • Consisting of all alleles present.

• Phenotype

  • Observable features of an organism.

  • eg. appearance or visible character.

• Heterozygous

  • An organism containing two different alleles of a particular gene.

  • If two separate alleles are passed down, e.g. the mother has blue eyes and the father has brown eyes, the offspring will have two different alleles for the gene, which is called heterozygous.

• Diploid nucleus

  • Nuclei which contains a full set of chromosomes (23 pairs).

• Haploid nucleus

  • Nuclei which only contain half the number of chromosomes.

  • These cells are egg and sperm cells, which fuse during fertilization to produce diploid cell.

Mitosis

• Is a form of cell division.

• Nuclear division of a parent cell occurs, producing two genetically identical daughter cells.

• Is used to create new cells in the body to repair and replace old and damaged tissues, as well as allowing growth of the organism and playing a role in asexual reproduction.

• Involves the splitting of chromosomes into their two halves, each of which are known as a chromatid.

Meiosis

• Is used to make four genetically unique daughter cells.

• Is used in the production of gametes.

• The chromosome number is halved, and a diploid cell divides to produce four haploid cells. As each gamete produced is genetically unique, each of the offspring will also be unique.

• Is beneficial for a species as it produces genetic variation.

Monohybrid Inheritance

Is the inheritance involving only one pair of contrasting characters.

Test cross

Through test cross the genotype of an organism showing the dominant trait can be determined by crossing it with an organism that is homozygous recessive.

• The organism is homozygous dominant, then all the offspring should show the dominant trait.

• If it is heterozygous, then half the total number of offspring should show the dominant trait, the remaining half, the recessive trait.

• For example;

  

Sex determination

There are 23 pairs of chromosomes in the nucleus of all diploid human cells. One of these pairs determine gender. These chromosomes are therefore called the sex chromosomes. For females these chromosomes are XX and for males the chromosomes are XY. All female egg cells contain only an X chromosome, whereas male sperm cells may contain an X or a Y, thus the gender of the baby depends on which sperm cell fertilizes the egg cell.

Mutations

Is the sudden or spontaneous change in gene structure or a chromosome, or even the chromosome number, or it can be inheritable.

• eg. of mutations

  • albinism

  • sickle cell anaemia

  • chromosome mutation

Discontinuous and Continuous Variation

Natural Selection and Artificial Selection

Genetic Engineering

Is a technique used to transfer genes from one organism to another.

• Advantages of genetic engineering

  • Can insert a gene from any organism into a plant or an animal.

  • In selective breeding, healthy and defective genes may be transmitted together to the offspring. This will not happen when genes are carefully selected before they are inserted into an organism.

  • Selective breeding is a slow process and may require a large space or land. Genetic engineering experiments with individual cells, which can reproduce rapidly in the laboratory in a small container.

  • Increases productivity and efficiency in the breeding of organisms.

• Risks

  • People may be allergic to the transgenic foods which they eat.

  • Genes that code for an antibiotic resistance may accidentally be incorporated into bacteria that cause diseases to humans.

  • Some people may deliberately create new combination of genes which they may use in chemical or biological warfare.