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Inheritance (3.14-3.21)

Important Definitions

Inheritance: the transmission of genetic information from generation to generation

Chromosomes: A thread of DNA made up of genes

Allele: Alternative version of the same gene

Gene: short length of DNA found on a chromosome that codes for a specific protein

Haploid nucleus: Nucleus containing a single set of unpaired chromosomes (23 in humans)

Diploid nucleus: Nucleus containing pairs of chromosomes (23 pairs in humans/ 46)

Genotype: The combination of alleles that control each characteristic

Phenotype: The observable characteristics of an organism

Homozygous: Pair of identical alleles control the same characteristic

Heterozygous: Pair of dissimilar alleles for a characteristic

Sex Inheritance

One of the 23 pairs of chromosomes codes for sex inheritance

Sex inheritance depends on the presence of X and Y chromosomes

  • Males have one X chromosome and one Y chromosome (XY)
  • Females have two X chromosomes  (XX)

→ Because only a father can pass on a Y chromosome, he determines the child's sex

  • a genetic diagram (Punnett square), with the X and Y chromosomes can be used to show the inheritance of sex

Protein Synthesis:

The base sequence on DNA codes for which amino acids should be joined in what order, to build a particular protein

  1. Proteins are made by ribosomes with the sequence of amino acids controlled by the sequence of bases present in DNA.

  2. DNA can't get from the nucleus to the ribosome , so the basic code for each gene is transcribed onto an RNA molecule called messenger RNA (mRNA).

  3. The mRNA then leaves the nucleus and attaches to the ribosome

  4. Ribosomes "read" the code on the mRNA in groups of three

  5. Each set of three bases codes for a specific amino acid The ribosome converts the base sequence into the amino acid sequence that makes up the protein

  6. Once the amino acid sequence has been assembled, it is released from the ribosome so that it can fold and form the final protein structure

→DNA controls cell function by controlling the production of proteins

The proteins may be enzymes, antibodies, or receptors for neurotransmitters

Cell Division

Mitosis: Nuclear division giving rise to genetically identical cells

  • Growth
  • Repair
  • Cell replacement

Diploid cell (46 chromosomes) undergoing mitosis:

  1. Each chromosome in the nucleus copies itself exactly (forms x - shaped chromosomes)

  2. Chromosomes line up along the centre of the cell where cell fibers pull them apart

  3. The cell divides into two; each new cell has a copy of each of the chromosomes

Meiosis: Nuclear division giving rise to cells that are genetically different. This is a reduction division to form haploid cells which produce gametes.

Diploid cell (46 chromosomes) will form haploid cells with 23 chromosomes.

  • Each chromosome makes identical copies of itself (forming X-shaped chromosomes)
  • First division: chromosomes pair up along the centre of the cell, recombination occurs and then cell fibres will pull the pairs apart, each new cell will have one of each recombinant chromosome pair
  • Second division: chromosomes will line up along the centre of the cell, cell fibres will pull them apart (as with mitosis)

A total of four haploid daughter cells will be produced

Monohybrid Inheritance \n

Monohybrid inheritance: the inheritance of characteristics controlled by a single gene (mono = one)

  • Dominant allele - Allele that is always expressed if present 

  • Recessive allele - Allele that is only expressed if the dominant allele is not present

 A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype

A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype. \n →An individual could be homozygous dominant, or homozygous recessive or heterozygous

1 recessive and 1 dominant; Dominant is expressed

Both recessive; Recessive is expressed

Both Dominant; Dominant is expressed

Punnet square diagram: Shows the possible combinations of alleles that could be produced in the offspring

Uppercase letters represent dominant alleles while lowercase letters represent recessive alleles

Ratio of Brown:Blue is 1:1

Codominance

Sometimes, neither of a pair of alleles is completely dominant or completely recessive

Codominance: Both alleles have an effect on the phenotype \n

Blood cells have antigens on their surface

The body will produce antibodies when foreign antigens are detected

Blood Types are an example of codominance

(Each letter represents the antigens on the bloods surface)

Type A has A antigens, Type B has B antigens, Type AB has both A & B antigens, Type O has NO antigens on its surface

There are three alleles of the gene governing this instead of the usual two

  • Alleles IA and IB are codominant, but both are dominant to IO
  • I represents the gene and the superscript A, B and O represent the alleles
  • IA results in the production of antigen A in the blood
  • IB results in the production of antigen B in the blood
  • IO results in no antigens being produced in the blood

This is important when finding compatible donors for blood transfusion

Sex-Linked characteristics

There are only alleles on X chromosomes

Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions 

As females inherit two copies of the X chromosome, one dominant allele masks the recessive one

Carriers are females who do not show the characteristic, but have a 50% chance of passing it on to their offspring.

→If that offspring is a male, he will show the characteristic