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What you need to know
Reflections and Exam tips
New cells come from pre-existing cells by a process of cell division- so says the Cell Theory.
But then- if this is so, where did the first cell come from? This is more like a chicken and egg relationship!
The origin of cells can be traced back to evolution.
Aghhh, the chicken and egg puzzle solved!
Types of cell division:
Before we talk about these, it is important to know these basic facts:
This is duplication division, as this suggets, it just copies a cell and makes another copy (clone) of the cell.
This is reduction division
This reduces the DNA content of the cell by half. Meiosis happens in reprodudctive organs that produce sex cells, in animals, testes (sperm) and ovaries for eggs (not penis and vagina), in plants in anthers (pollen) and ovaries (ovules/eggs).
Genetics is the study of inheritance, how characteristics are passed on from parent to offspring.
Genes: genes are units of inheritance, parts of chromosomes which are responsible for passing characteristics.
The guy who developed our understanding of inheritance. He is sometimes called the father of genetics.
Mendel's work demonstrates the patience and virtues of being a scientist. He carried out long, time consuming experiments with peas, looking at how features like height, seed shape and flower colour are passed on.
Before we explore his experiments, we can look at a few key definitions:
For each characteristic, we inherit 2 alleles (so we gonna have 2 letters), one from mama (martenal), one from papa (partenal), which ever comes out in our phenotype depends on which one is dominant/recessive.
Let us look at some of Mendel's works:
Mendel crossed pure breeding (homozygous) tall plants with pure breeding (homozygous) Short plants
Tall x Short
The resulting first generation (also called F1 generation) were all tall!
Tall x Short
Why were they all tall? mendel asked!
Mendel then crossed these tall F1 plants together to get an F2 Generation
The result 3 Tall 1 Short
These can be worked out using a punnet square, more or less like a Maths multiplication table:
F1 Tall (HH) x Short (hh)
F2 Tall (Hh) x Tall (Hh)
F2 generation 3 Tall: 1 Short
Most features are Inherited this way.
Inheritance of gender/sex
We have 2 sex chromosomes numbers 23,
Cystic Fibrosis (CF)*
Cystic Fibrosis is a disease that affects inner membranes of our body.
Its caused by a recessive gene (allele) which results in the shortage of a key membrane protein. This results in excessive production of mucus in the Breathing and digestive systems. The mucus affects gaseous exchange, absorption of food and may encourage the growth of infectious bacteria.
As with all conditions caused by recessive genes, you need 2 recessive Alleles to be affected i.e. cc. If you are heterozygous Cc, you are a carrier of the condition. You have the allele but you are not affected. You may pass the condition to your kids.
Normal people have CC.
Try doing a punnet square to check the probability of a child inheriting it from 2 carrier parents.
Polydactyly is having more than 5 fingers and toes on each limb.
It is caused by a Dominant allele, so you just need to inherit 1 of them to be affected. (both PP and Pp are affected)
Normal people are recessive pp
Again, practise genetic crosses to show how this is inherited.