Gregor Mendel's monohybrid cross
- Mendel noticed that all pea plants were different - size, pea colour, pea shape.
- He cross-mated them to study how a single characteristic was passed on.
- We know now that these characteristics were caused by the alleles on the chromosomes being passed on-depending on the parent alleles passed on.
Alleles and Pea Plants
- Usually one allele will be dominant over the other and in order for the recessive allele to be seen in the phenotype (physical appearance) of the organism. It must be present on both chromosomes off the offspring - be homozygous recessive .
- Before selecting his parent plants, Mendel often let them breed until all offspring that were produced, showed the same characteristic - these parent plants were then called pure breeding for that characteristic.
Sex Cells and Chromosomes
Human boy cells each contain 23 pairs of chromosomes. Parents pass on their genes in their sex cells (ovum in females, sperm in males). Each sex cell contains 23 separate chromosomes which, when they join in fertilisation, creates 23 pairs of chromosomes - a new being with brand new cells. Each pair of chromosomes carries the same genes in the same places.
Which chromosome we get from each pair of our parents' chromosomes is completely random. This is why we look a little bit like our parents and our siblings, but we are not identical to them.
Females have XX chromosomes - all X
Males have XY chromosomes