1. Introduction to Heredity
DNA segments make genes.

4. Chromosomes
Chromosomes come in 2 forms, depending on the stage of the cell cycle. The monad form consists of a single chromatid.The dyad form consists of 2 identical chromatids (sister chromatids) attached together at the centromere.
Chromosomes are in the dyad form before mitosis begins, and in the monad form during interphase.
The dyad form is the result of DNA replication: a single piece of DNA (the monad chromosome) replicated to form 2 identical DNA molecules (the 2 chromatids of the dyad chromosome).

5. More Chromosomes
Diploid organisms have 2 copies of each chromosome, one from each parent. The two members of a pair of chromosomes are called homologues.
Each species has a characteristic number of chromosomes, its haploid number n. Humans have n=23, that is, we have 23 pairs of chromosomes. Drosophila have n=4, 4 pairs of chromosomes.

6. Homologous Chromosomes
Same length
Same centromere position
Carry genes that control the same inherited traits

7. The number of chromosomes in the body cells is called 14
The number of chromosomes in the body cells is called
the diploid number
The number of chromosomes in the gametes is called
the haploid number (half the diploid number)
The diploid number in human cells is 46. The haploid
number in the gametes is 23

8. Meiosis – making haploid sex cells to create the new offspring

10. Meiosis Cartoon Meiosis Square Dance
Meiosis Square Dance

11. ova produced by meiosis but only one develops to maturity15 23
sperms produced
by meiosis 46
sperm
mother
cell
fertilization
zygote 23 46
ovum
mother
cell 23 23
Human body cells contain 46 chromosomes, but the gametes contain only 23.
At fertilisation, the number is restored to 46.
In humans and most other mammals, the ovum mother cell produces four cells by meiosis but only one of these goes on to become a gamete.
ova produced by meiosis
but only one develops to
maturity

13. Cell division continues by mitosis, so all the cells will 16
Cell division continues by
mitosis, so all the cells will
contain 46 chromosomes
early embryo 46

14. Human Karyotype – a picture of our 46 chromosomes

16. Meiosis has a special twist Crossing Over = Genetic Recombination
Purpose: to create chromosomes that are a new combination of paternal and maternal material
Genetic variation is important to the survival of every species.
Which genes are separated more frequently??? Ones that are close or far apart?

18. Meiosis Clicker Check for Understanding

19. What does Meiosis have to do with Inheritance or Genetics?
Law of Segregation - during the production of gametes the two copies of each hereditary factor (alleles) segregate so that offspring acquire one factor from each parent
Law of Independent Assortment - Genes passed to offspring that code for a particular trait are not linked to different genes which code for different traits. ie. The gene for brown eyes assorts independently from the gene for freckles. (possible because of crossing over OR genes are on different chromosomes)
There are exceptions – genes that are very close on the same chromosome can be “linked”: blonde hair & blue eyes

20. All offspring will be black (Bb) meiosis fertilization meiosis
sperm mother cell B b b
ovum mother cell b
zygote
meiosis b

21. 8
The offspring from this cross are called the F1 (First Filial) generation
They are all black because the allele for black coat color is
dominant to the allele for brown coat colour
These Bb mice are called heterozygotes. Because the B and b
alleles have different effects; producing either black or brown coat
color The mice are heterozygous for coat colour
The BB mice are called homozygotes because the two alleles
produce the same effect. Both alleles produce black coats.
The bb mice are also homozygous for coat colour. Both alleles
produce a brown coat color
The next slide shows what happens when the two heterozygotes
are mated and produce young

22. F2 9 Fertilization Possible combinations sperms sperm mother cellBB b B b
sperm mother cell Bb
meiosis
ovum mother cell B B b B b Bb b b
ova bb
zygotes

23. 10 Punnett square B b female gametes B male gametes b
A neater way of working out the possible combinations
is to use a Punnett Square*
Draw a grid
Enter the alleles in the gametes
Enter the possible combinations B b
female gametes B
* After Dr. R.C. Punnett, Professor of Genetics in Cambridge in the first half of the 20th century. BB Bb
male
gametes b
These are the
F2 generation Bb bb