Inheritance.

Inheritance.

Sex, or gender, is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a few pairs of genes) Females have the sex chromosomes XX  Males have the sex chromosomes XY

Determination of Sex

• As only the father can pass on either an X or Y chromosome, they are responsible for determining the sex of the child

• The inheritance of gender can be shown using a genetic diagram ( known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes

Multiple Choice

What is the correct term for the variant form of a gene?

• allele

• alelle

• gene

• DNA

Multiple Choice

How many PAIRS of chromosomes are present in a human karyotype?

• 46

• 23

• 26

• 43

Cell Division - Mitosis

• Most body cells have two copies of each chromosome We describe these cells as diploid

• When cells divide their chromosomes double beforehand This ensures that when the cell splits in two, each new cell still has two copies of each chromosome (is still diploid)

• This type of cell division is used for growth and repair in the body and is known as mitosis

The process of cell division by mitosis

• Process:

  Just before mitosis, each chromosome in the nucleus copies itself exactly (forms x – shaped chromosomes)

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

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

Importance:

• All cells in the body (excluding gametes) are produced by mitosis of the zygote

• Mitosis is important for replacing cells e.g, skin cells, red blood cells and for allowing growth (production of new cells e.g. when a zygote divides to form an embryo)

Occurs in:

• Growth: mitosis produces new cells

• Repair: to replace damaged or dead cells

• Asexual reproduction: mitosis produces offspring that are genetically identical to the parent

Multiple Choice

What is a zygote?

• fertilised egg cell

• sperm cell

• muscle cell

Open Ended

Humans are described as diploid organisms. What does this mean?

Open Ended

what does mitosis prduce?

Open Ended

What does mitosis occur in?

Open Ended

Why is mitosis important?

Open Ended

During asexual reproduction why mitosis produce offspring that are genetically identical to the parent?

Meiosis

• The number of chromosomes must be halved when the gametes (sex cells) are formed

• Otherwise there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)

• This halving occurs during a type of cell division called meiosis

• It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell

Meiosis

• After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete

• We describe gametes as being haploid – having half the normal number of chromosomes

• Because of this double division, meiosis produces four haploid cells

The process of cell division by meiosis to produce haploid gamete cells

• 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

Importance:

• Production of gametes e.g. sperm cells and egg cells, pollen grains and ovum

• Increases genetic variation of offspring

• 
  

  Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others

Multiple Choice

What does haploid mean?

• two copies of each chromosome

• half the normal number of chromosomes

• twice the amount of the normal number of chromosomes

Open Ended

Why is meiosis important?

Open Ended

Why is genetic variation in a species important?

Multiple Choice

How many cell divisions occur in mitosis?

• 1

• 2

• 3

• 4

Multiple Choice

How many cell divisions occur meioses?

• 1

• 2

• 3

• 4

Multiple Select

Daughter cells are genetically identical to each other and the parent cell

• Meiosis

• Mitosis

Multiple Choice

Daughter cells are genetically different from each other and the parent cells

• meiosis

• mitosis

Multiple Select

Daughter cells are diploid

• meiosis

• mitosis

Multiple Select

Daughter cells are haploid

• Mitosis

• Meiosis