AQA GCSE Biology - Inheritance

Gametes

sex cells

Gametes are made by

Meiosis

Meiosis

Produces non identical cells

Sexual reproduction

-Involves the fusion of Male and female gametes (fertilisation)
-mixing of genetic information so we can see variation in offspring

Asexual reproduction

-one parent
-does not involve gametes and no mixing of genetic information
-offspring are identical
-only involves mitosis

Clones

identical genetic copies

Meiosis

-all chromosomes are copied
-cell divides into two and then one more time, forming gametes
-in the gametes the chromosomes are not in pairs so they have been halved
-produces four gametes from one cell, all genetically different

Meiosis takes place

In reproductive organs, ovaries and testes

Fertilisation

Fusing of a male sex cell with a female sex cell

After fertilisation

The cell divides by mitosis and produces a clump of identical cells (embryo)

As an embryo develops

The cells differentiate forming different cell types eg muscle and nerve cells

Advantages of sexual reproduction

-variation which gives the species more chance of survival because of natural selection (some will survive and reproduce)

Advantages of asexual reproduction

-no need to find a mate
-more efficient and faster
-allows an organism to produce many genetically identical offspring rapidly

Disadvantages of asexual reproduction

-no genetic variation so if conditions become unfavourable they all die

Malaria

-in the human host, parasite reproduces asexually
-in the mosquito, parasite reproduces sexually

Fungi

-produce spores to reproduce asexually
-can also reproduce sexually to create variation

Flowering plants

-reproduce sexually to produce seeds
-strawberry plant reproduces asexually using runners, when it touches the soil it develops into a new plant

Daffodils

-reproduce asexually via bulb division
-parent plant has an underground bulb which produces buds
-these buds eventually form new offspring which are genetically identical

Chromosomes contain

DNA

Structure of DNA

double helix, each strand is a polymer

DNA contains

genes

Each gene

encodes for a specific sequence of amino acids to make a unique protein

Genome

all of an organism's genetic material

Benefits of understanding the human genome

-help us to search for genes that are linked to a disease like cancer
-help us treat inherited disorders like CF
-trace human migration patterns to discover ancestors

DNA structure

polymer of four nucleotides

nucleotides

contain a phosphate group, sugar molecule, base

What doesn't change in a nucleotide

the phosphate group and sugar molecule

Bases

complementary and there are four of them

Base pairs

A-T
C-G

Proteinsynthesis

glucose is used to make amino acids for making proteins

Most proteins contain

all 20 amino acids

What does the order of amino acids determine?

the shape of the protein

What does the shape of a protein determine?

its function

Examples of proteins

enzymes and antibodies

How is the order of amino acids determined?

by a sequence of bases in the gene for that protein

Transcription

-the organic process whereby the DNA sequence in a gene is copied into mRNA
-passes out of the nucleus into the cytoplasm

Translation

-mRNA is attached to a ribosome
-amino acids are brought to the ribosome on carrier molecules (transfer RNA)
-ribosomes reads the triplets of bases on the mRNA and uses this to join together the correct amino acids in the right order
-when chain is complete, folds into unique shape

Mutation

A change in DNA structure

Why do some mutations have no effect?

Because different base triplets can sometimes encode for the same amino acid

If an enzyme has a mutation

The substrate may no longer fit into the active site

If a structural protein has a mutation

It may lose its strength

Chromosomes also contain

Non-coding parts of DNA

Gregor Mendel

investigated products of sexual reproduction on pea plants

What did Mendel discover?

-he discovered that characteristics are determined by 'units' that are inherited and do not blend together
-scientists discovered that the 'units' he discovered acted similarly to chromosomes and they were later named genes

Why was Mendel's discovery not recognised in his lifetime?

-he was a monk not a scientist
-he did not publish his work in a well known book

Alleles

Versions of a gene

Genotype

genetic makeup of an organism

Homozygous

Having two identical alleles for a particular gene

Heterozygous

Having two different alleles for a trait

Phenotype

An organism's physical appearance, or visible traits.

Recessive

A recessive allele will only show in the phenotype if two copies are present

Dominant

A dominant allele will show in the phenotype even if there is only one copy present

Cystic fibrosis

-disorder of cell membranes
-controlled by a single gene
-defective allele is recessive
-affects respiratory and digestive system

What is a carrier

heterozygous genotype that carries recessive trait but doesn't express it

Polydactyly

-extra fingers or toes
-caused by a dominant allele

You cannot be a carrier of

A dominant allele because you will have the phenotype

Embryo screening

A process of testing embryos for genetic disorders and rejecting embryos that are at risk

Issues around embryo screening

-expensive
-some healthy embryos are destroyed

Gene therapy

The insertion of working copies of a gene into the cells of a person with a genetic disorder in an attempt to correct the disorder

Family trees show

Phenotypes

How do we know cystic fibrosis is caused by a recessive allele by looking at a family tree?

A person has it but neither of their parents do, so they must both be carriers of the recessive allele