Variation, Inheritance & Environment

Variation

• Differences between members of the same species are called variations.
• Variations arise for different reasons.

Inheritance

• Genes inherited from parents are responsible for variations.
• These variations are inherited, including eye color, blood group, and skin color.

Environment

• Some variations are a result of differences in the environment
• Differences in the environment can cause variations after fertilization such as scars and accent.

Inheritance and Environment

• Certain features are due to a combination of inherited and environmental causes.
• For example, tall parents may pass genes for height
• Children have a genetic potential to be tall.
• However, if their diet is poor, then they will not grow as well.
• The environment also has an impact on their height.
• Weight and skin tone also fall into this category.

Investigating Variation

• When measuring variations in a population, data ends up being continuous and discontinuous.
• Continuous variation has no distinct categories and is influenced by the environment.
• Examples consist of height and foot length.
• Discontinuous variation displays distinct categories and is not influenced by the environment.
• Examples are eye colour, gender, blood type and ability to roll tongue.

Why is variation important?

• Changes in the environment, like a loss of food source, might make it difficult for a species to survive.
• If variation exists within the population, some individuals may be able to cope with the changes.
• Survival advantage causes these individuals to be more likely to survive and reproduce avoiding becoming extinct.

What exactly is DNA?

• The long chain molecule called deoxyribonucleic acid (DNA) comprised of joining lots of smaller molecules together.
• Smaller molecules that makeup DNA are called nucleotides.
• A nucleotide's base contains nitrogen.
• Each nucleotide contains adenine, thymine, cytosine and guanine.

What exactly is DNA?

• Each different color represents a different nucleotide.
• Nucleotides join to form chains.

What exactly is DNA?

• Each different color represents a different nucleotide.
• Nucleotides join to form chains
• These chains form helixes.
• DNA rarely exists as a single chain.

The double helix

• Two chains join to form the structure known as the double helix.
• The two chains are held together by bonds between the bases.
• Bases in the nucleotides always pair up in the same way: Adenine (A) - Thymine (T), Guanine (G) - Cytosine (C).

What are genes?

• Genes are sections of DNA that code for proteins.
• Proteins are little machines that do jobs for our cells.
• There are thousands of different proteins doing different jobs, for example, haemoglobin that carries oxygen.

What exactly are chromosomes?

• DNA forms very long strands called chromosomes.
• Each chromosome has thousands of genes.
• Most human cells contain 46 chromosomes (23 pairs).
• Chromosomes are found in the nucleus.

Alleles

• Alleles are different versions of the same gene with two alleles for each feature (e.g., eye colour), one from each parent.
• When both alleles in the pair are the same, this is called homozygous.
• When the alleles in the pair are different, this is called heterozygous.
• A dominant allele (shown with a capital letter) is always expressed, even if only one copy is present.
• A recessive allele (shown with a lower case letter) is only expressed if the individual has two copies and does not have the dominant allele of that gene.
• The combination of alleles an individual has is called their genotype.
• The characteristics they have due to their genotype are called their phenotype.

Example

• Imagine that the brown eye phenotype is controlled by a dominant allele shown as 'B' while the blue eye phenotype is controlled by a recessive allele shown as 'b'.
• An individual with the genotype BB is homozygous for the dominant brown eye allele and would have brown eyes, which is their phenotype.
• An individual with the genotype Bb is heterozygous and has one copy of the dominant brown eye allele, therefore, they would have brown eyes, which is their phenotype.
• An individual with the genotype bb is homozygous and has both copies of the recessive blue eye allele, therefore, they would have blue eyes, which is their phenotype.

Genetic Crosses

• Genetic crosses are used to work out the different offspring that are possible from two specific parents.
• One way of doing this is to use the punnet square method.

Example

• Coat type in dogs is determined by a single gene with two alleles.
• Straight coat allele is dominant (D) and the curly coat allele is recessive (d).
• 50% (2 out of 4) chance exists that an offspring will have the genotype Dd and have the straight coat phenotype.
• 50% (2 out of 4) chance exists that an offspring will have the genotype dd and have the curly coat phenotype.

Inheritance of Gender (sex)

• Chromosome pair 23 determines gender (sex).
• Chromosome 23 can be an X or a Y (they have been given these names because they have a different shape and size).
• Individuals that are XX (so both chromosomes in pair 23 are an X) are female.
• Individuals that are XY are male.
• Females produce egg cells that carry an X chromosome.
• When a man produces sperm, 50% will carry an X chromosome that lead to a female offspring.
• 50% of the sperm produced by a man will carry a Y chromosome and lead to a male offspring.

Genetic Screening

• Genetic screening involves the analysis of a person's DNA to see if they carry alleles that cause genetic disorders.
• Pre-implantation genetic diagnosis (PGD) can be used as part of the in vitro fertilisation (IVF) process.
• IVF is when fertilisation is done by scientists in a laboratory using sperm and eggs collected from a male and female.
• PGD involves testing the embryos that are made as a result.
• Embryos that don't contain any genetic disorders are implanted into the uterus.

Limits of Genetic Testing

• Genetic tests are not available for every possible inherited disorder, and are not completely reliable.
• May produce false positive or false negative results, which can have serious consequences for the parents involved.

False positives

• A false positive is a genetic test that wrongly detected a certain allele or faulty chromosome.
• The individual could believe that they have inherited a genetic condition, when they have not.

False negatives

• A false negative is a genetic test that has failed to detect a certain or faulty chromosome.
• The parents may be given incorrect results.
• These results can have an impact on the lives of individuals, such as planning the level of care needed for children with inherited disorders.

Selective Breeding

• Selective breeding is the process by which humans choose animals to breed together to achieve a desired characteristic in the offspring.
• Characteristics can be chosen for usefulness or appearance.
• Example - Disease resistance in food crops.
• Animals which produce more meat or milk.
• Domestic dogs with a gentle nature.
• Large or unusual flowers.

Selective Breeding Process

• Choose parents with the desired characteristic from a mixed population.
• Breed the chosen parents together.
• From the offspring choose those with the desired characteristic are bred together.
• This continues over many generations until all the offspring show the desired characteristic.

Problems with Selective Breeding

• Selective breeding reduces variation in populations.
• This makes it difficult to produce different varieties in the future.
• Selective breeding can also lead to health problems.

Genetic Engineering

• Genetic engineering is the deliberate modification of an organism's genes.
  • GE = Genetic engineering
  • GM = Genetically modified (organisms which have been genetically engineered)
  • GMO = Genetically modified organism
• Crops can be given extra genes for new and useful characteristics:
  • Examples of traits include pest resistance, frost resistance, disease resistance, herbicide resistance, drought resistance, and longer shelf life.

Cloning

• A clone is a genetically identical copy of another organism.
• The first organism to be cloned was a sheep called Dolly that was born in 1996.