Inheritance, variation & evolution flashcards

Key points

• DNA is a polymer made of two strands twisted into a double helix.
• In the nucleus, DNA is coiled into chromosomes. Most human body cells have 23 pairs.
• A gene is a small section of DNA that codes for a particular sequence of amino acids, to make a specific protein.
• The genome is the entire genetic material of an organism.
• Studying the human genome helps find genes linked to disease, treat inherited disorders and trace human migration.

Key points

• Alleles are different versions of a gene. You inherit one allele of each gene from each parent.
• Dominant (B): shows with only one copy. Recessive (b): shows only with two copies (bb).
• Homozygous: two identical alleles (BB or bb). Heterozygous: two different alleles (Bb).
• Genotype: the alleles present, such as Bb. Phenotype: the characteristic shown, such as brown eyes.
• Most characteristics come from several genes interacting, not from one gene alone.

Key points

• Write one parent's alleles across the top and the other parent's down the side.
• Fill each box with one allele from the top and one from the side.
• Each box is a 1 in 4 chance for each child.
• Count the boxes to give the probability or the ratio of each phenotype.

Bb × Bb, brown (B) dominant to blue

Key points

• Cystic fibrosis is a disorder of cell membranes caused by a recessive allele (f).
• It needs two copies (ff). Someone with one copy (Ff) is a carrier: they do not have it but can pass it on.
• Polydactyly, extra fingers or toes, is caused by a dominant allele (D), so one copy is enough.
• Embryos can be screened for these alleles, which raises ethical, social and economic questions.

Polydactyly: Dd × dd

Key points

• One of the 23 pairs of chromosomes is the sex chromosomes.
• Females are XX; males are XY.
• Every egg carries an X. Half of all sperm carry an X and half carry a Y.

XX × XY

Key points

• Gametes are sex cells: sperm and eggs in animals, pollen and eggs in plants.
• Meiosis makes gametes, in the reproductive organs such as the testes and ovaries.
• The chromosomes are copied, then the cell divides twice, making four gametes, each with one set of chromosomes.
• The four gametes are all genetically different.
• At fertilisation two gametes join, so the new cell has the full number again. It then divides by mitosis.

Key points

• Sexual: two parents, and gametes made by meiosis join at fertilisation.
• It mixes genetic information from both parents, so the offspring vary.
• Asexual: one parent, no gametes and no mixing of genetic information.
• It uses mitosis, so the offspring are genetically identical clones of the parent.

Key points

• Variation is the differences in characteristics between individuals of the same species.
• It is caused by genes, the environment, or both: eye colour (genes), a scar (environment), height (both).
• There is usually a lot of genetic variation within a population.
• New alleles come from mutations, changes in DNA. Most have no effect; rarely one gives a feature that suits the environment.

Key points

• Individuals in a population vary, and some of the variation is inherited.
• Those best suited to the environment are more likely to survive and reproduce.
• They pass on their alleles, so those alleles become more common in the next generation.
• Over many generations the characteristics of the population change.

Key points

• Evolution is a change in the inherited characteristics of a population over time, through natural selection.
• Charles Darwin set out the theory of evolution by natural selection in On the Origin of Species (1859).
• All living things have evolved from simple life forms that first appeared more than three billion years ago.
• Two populations are separate species when they can no longer interbreed to produce fertile offspring.

Key points

• Fossils are the remains of organisms from millions of years ago, found in rocks.
• They form from parts that did not decay, from parts replaced by minerals, or as traces such as footprints and burrows.
• Fossils in the rock layers show how organisms have changed over time.
• The record has gaps: early life was soft-bodied and left few traces, and geological activity destroyed many fossils.
• Bacteria reproduce so fast that resistance to antibiotics evolves quickly enough to watch.

Key points

• People choose parents with a wanted characteristic and breed them together.
• From the offspring, those with the characteristic are bred again, over many generations.
• It gives crops that resist disease, animals with more meat or milk, gentle dogs and large or unusual flowers.
• Risk: inbreeding reduces variation, so some breeds are more prone to diseases or inherited defects.

Key points

• Genetic engineering changes an organism's genome by inserting a gene from another organism, giving a useful characteristic.
• Bacteria given the human insulin gene make insulin for people with diabetes.
• GM crops can resist insect attack or herbicides, giving bigger yields.
• Concerns: effects on wild flowers and insects, and the long-term effects of eating GM crops are not fully known.

Key points

• Enzymes are used to cut out the required gene.
• The gene is inserted into a vector, usually a bacterial plasmid or a virus.
• The vector carries the gene into the required cells.
• In plants and animals the gene goes in at an early stage of development, so the organism grows with the new characteristic.

Key points

• Linnaeus sorted living things into groups by their structure and characteristics.
• The groups run kingdom, phylum, class, order, family, genus, species, from largest to smallest.
• A binomial name is the genus then the species, such as Homo sapiens.
• Better microscopes and chemical analysis led Carl Woese to propose three domains: archaea, bacteria and eukaryota.