gamete = sex cell; sperm or egg; these cells are haploid (n)

zygote = fertilized egg; these and all body cells are 2n

Humans have 46 chromosomes

But if two people have a baby, and each gives ALL their chromosomes to the baby, then the baby would have 92 chromosomes.

In the next generation, there would be 92 + 92 = 184 chromosomes.This is too many!

So instead, the sex cells go through meiosis to reduce the number of chromosomes to half (from 2n to n)

Meiosis (said my-o-sis) = reductional division

meiosis is similar to mitosis, going through interphase, prophase, metaphase, anaphase and telophase, but there are differences as well.You will need to MASTER this table - don=t just know it, but UNDERSTAND it!This is also discussed in your textbook .

 

Mitosis	Meiosis
1 nuclear division	2 nuclear divisions
1 cellular division	2 cellular divisions
parent is diploid, daughters are diploid	parent is diploid, daughters are haploid (chromosome number is halved)
daughters identical to each other and parent	daughters different from each other and from parent
chromosomes duplicated exactly	chromosomes duplicated exactly, but later modified
occurs in many cell types throughout the organism	occurs only in reproductive cells to form gametes

Be comfortable with all the terms in this list as well as the information in the rest of this handout; use your textbook to look up terms you don=t understand, and come to class prepared with questions.

Gene - piece of DNA that codes for a protein

Gene locus - location, or address, on a chromosome where a particular gene resides

Homologous chromosomes - two chromosomes, one that came from your mom, one from your dad, that contain the same types of information (for instance, both contain information on your hair color, eye color, whether you like to read, etc.)But the information at one gene locus may be the same or it may be different

alleles - two pieces of DNA at a given gene locus that contain DIFFERENT information; for instance, one allele for blue eyes, another for brown eyes

homozygous - if the two alleles at a given gene locus are the same - for instance, you may have two alleles for blue eyes, one from your mom and one from your dad

heterozygous - if the two alleles at a given gene locus are different - for instance, one allele for blue eyes and one allele for brown eyes

dominant allele - this allele is expressed if you have one or two copies of it; for instance, if you have a brown eye allele, it is always expressed - whether you have two copies (homozygous) or one brown and one blue allele (heterozygote).Dominant traits are symbolized by a capital letter (e.g. B for brown eyes, F for free earlobes)

recessive allele - this allele is only expressed if you have two copies of it (homozygous); but in the heterozygote it is >hidden=.Recessive traits are symbolized by a lower case letter of the same type as the dominant allele (e.g. b for blue, f for attached earlobes).Note that the small letter is of the same type as the letter for the dominant allele (not a for attached earlobes, but f, because the dominant trait is F for free).

I have a blue eye allele from my mom, but because I got a brown eye allele from my dad, my eyes are brown - the blue eye allele is hidden.So I am a heterozygote, and brown eyes are dominant to blue eyes.

Putting it all to work: 

These two chromosomes are homologous: they each have four genes.

This individual is homozygous dominant for freckles (FF).There are two copies of the allele for the dominant trait.

This individual is homozygous recessive for no widow=s peak (ww).There are two copies of the allele for the recessive trait.

This individual is heterozygous for dimples (Dd).Dimples is the dominant trait.Does this person have dimples or no dimples? ________

The last gene is for tongue rolling.The tongue-rolling allele (T) is dominant to the allele for non-tongue rolling (t).This individual is ____________ for the trait.They can/can not (circle one) roll their tongue.