iGCSE Biology Notes Inheritance

[q] How is DNA organized?

[a] 

• DNA is stored in the nucleus of cells,and it’s the cells genetic information.
• A gene is a section of DNA that codesfor one protein.
• A chromosome is a long length ofDNA which carries genetic informationin the form of genes.

[q] Diploid

[a] 

• A diploid nucleus is a nucleus that contains two sets of chromosomes.
• This means 23 pairs (so 46 in total)
• All of your normal somatic cells are diploid

[q] Haploid

[a] 

• A haploid nucleus is a nucleus that contains one set of chromosomes.
• This means 23 chromosomes only.
• Gametes (sperm/egg cells) are haploid

[q] Mitosis 

[a] 

• Mitosis is a nuclear division giving rise to genetically identical cells
• Mitosis is needed for growth, repair of damaged tissues and cell replacement.
• It is also used for asexual reproduction in some organisms.

[q] Mitosis: The steps 

[a] 

• Step 1: Duplicate the chromosomes
• Step 2: Line up the chromosomes along the centre
• Step 3: Separate the chromosomes
• Step 4: Nucleus divides
• Step 5: Cytoplasm divides
• This creates diploid cells

[q] Stem cells

[a] 

• Stem cells are undifferentiated cells that divide asymmetrically by mitosis to produce specialized cells
• Almost all cells in the body have the same DNA, but they become specialized because some genes are switched on and some are switched off.

[q] Meiosis 

[a] 

• Step 1: Duplicate the chromosomes
• Step 2: Line up the chromosomes along the centre
• Step 3: Separate the chromosomes
• Step 4: Nucleus divides
• Step 5: Cytoplasm divides
• Step 6: Nuclei divide again
• Step 7: Cytoplasm divides again
• This creates haploid gametes

[q] How does this build on what you already know?

How does link to the rest of biology?

[a] 

• A gene is a section of DNA that codes for a protein
• These proteins could be anything including enzymes, hormones, neurotransmitters and cell membrane carrier proteins

[q] Let’s look closer – Transcription

[a] 

• Each three bases of DNA is called a triplet.
• An enzyme will come along, unzip the DNA strands so the bases are exposed, and transcribe a complimentary strand of mRNA.
• mRNA is a lot like DNA, but it uses the letter U instead of T.
• This process is called transcription.
• This is so DNA stays in the nucleus.

[q] Let’s look closer – Translation

[a] 

• Each three bases of mRNA is called a codon.
• The mRNA will move out of the nucleus into the cytoplasm, and attach to a ribosome.
• The ribosome will read the code and choose the correct amino acid for each codon.
• Each codon codes for a different amino acid, and there are 20 amino acids in total.
• The ribosomes assembles the amino acids into proteins.

[q] Overview

[a]

[q] What are alleles?

[a] 

• Every person has two versions of every gene.
• These versions of genes are called alleles
• Inheritance is the passing on of genes to the next generation

[q] There are two types of alleles

[a] 

• An allele is either dominant or recessive.
• If a person has a dominant allele, it will always be expressed
• If a person has a recessive allele, it will only be expressed if there is no dominant

[q] What are the combinations?

[a] 

• This means for each gene, you can either have 2 dominant alleles, 2 recessive alleles, or one of each.
• Homozygous dominant means you have 2 dominant alleles
• Homozygous recessive means you have 2 recessive alleles
• Heterozygous means you have 1 dominant and 1 recessive allele

[q] Genotype

[a] The genotype of an individual is whether they are homozygous dominant, homozygous recessive or heterozygous for a particular gene

[q] phenotype 

[a] The phenotype of an individual is the characteristic you can see in the organism

[q] Punnett squares

[a] We can use Punnett squares to predict the genotypes and phenotypes of the offspring when two individuals mate

[q] Sex inheritance

[a] 

• The 23rd pair of chromosomes are the sex chromosomes
• If you are female you have 2 X chromosomes
• If you are male you have 1 X and 1 Y chromosome
• This means males determine the sex of the offspring

[q] Genetic diagrams

[a]

[q] Pedigree diagrams

[a]

[q] What is codominace?

[a] Sometimes, the alleles for a gene can be codominant which means in a heterozygote, both alleles are expressed.

[q] What are blood groups?

[a]

• Blood groups can show codominance.
  
• The alleles are IA (for group A), IB (for group B) and I (for group O).

[q] Sex linkage

[a] 

• Sex linkage is where a gene is located on a sex chromosome.
• This means a trait or disease may be more likely to be passed on to a specific sex
• A disease can be X-linked or Y-linked

[q] Sex linkage

[a] 

• Red-green colourblindness is an Xlinked recessive disorder
• This means males are more likely to have it has they only have one X chromosome.