Mendel and Meiosis

Genetics  Study of heredity

Gregor Mendel  Monk  Studied pea plants- easy to grow, observable traits, easy to manipulate  Plants also reproduce sexually-produce gametes(sex cells)  Selected to breed tall plants(Parents-P) with short plants. Made hybrids  Hybrid- offspring of parents with different traits(F 1 )

 Next he bred the offspring with each other to get 2 nd generation(F 2 )  P-parents  F 1 -you  F 2 -your kids  What did Mendel figure out?

Mendel’s Findings  Each organism has 2 factors that control each trait. (GENES) 1 from mom,1 from dad  Genes exist in different forms called alleles-sort of like choices. Ex. Eye color- 2 choices or alleles Ex. Eye color- 2 choices or alleles BLUE or BROWN BLUE or BROWN

Rule of Dominance  One allele is dominant over the recessive allele.  Ex. Plant height- Tall allele(T) is dominant over short allele(t)  T-Dominant(Upper case)  t-recessive (lower case)

Law of Segregation  Every individual has 2 alleles for each trait(1from mom,1 from dad)  When that individual produces gametes(sex cells) each sex cell will only have 1 of the alleles for each trait  Ex. Plant could be TT or Tt or tt for its allelic combination but it would only donate one to a gamete. TT- T or T Tt- T or t Tt- T or t tt- t or t tt- t or t

Law of Independent Assortment  Each allele is inherited independently of other alleles  Ex. Pea seed color and shape are inherited independantly of each other so you can have: Smooth green peas, Wrinkled green peas Smooth yellow peas, Wrinkled yellow peas The more alleles for a trait leads to more possible combinations.

Genetics vocabulary  Phenotype- how the trait looks(observe) Ex- tall, short,blue eyes, brown eyes  Genotype- allelic combination Ex. TT,Tt,tt BB,Bb,bb  Homozygous- both alleles are the same Ex. TT,tt BB,bb  Heterozygous- allele combination is different Ex. Tt Bb

Punnett Squares  We can use a punnett square to predict the results of breeding between 2 parents. Sometimes our predicted results do not match the actual results.

 T t Heterozygous  Cross T Phenotype ratio-3:1 t Genotype ratio – t Genotype ratio – 1:2:1 1:2:1 Predicted results: Genotype Phenotype Genotype Phenotype 25% TT 75% Tall 25% TT 75% Tall 50% Tt 25% short 50% Tt 25% short 25% tt 25% tt TTTt tt

More Complex Alleles  Incomplete Dominance- neither allele is dominant. Red flowers X White flowers =Pink Flowers  Co dominance- both alleles expressed ex Black chicken X White chicken= Checkered chicken

Multiple alleles  more than 2 possible alleles but there can only be 2 in each individual. Ex.Pigeon color- grey,black, white, brown

Polygenic inheritance  many genes determine phenotype. Ex. Skin color and height

 We know that mitosis produces identical cells.  In humans each body cell has 46 chromosomes(23 pairs)  If we joined 2 cells each having 46 chromosomes what do we get?  A cell with 92 chromosomes-TOO MANY!!!

What to Do?  Somehow we need to have a cell with ½ the number of chromosomes(1/2 x 46=23) so that if 2 cells joined together each having 23 chromosomes we end up with the magic number of 46! (2 x 23)

 All organisms have a set number of chromosomes. They get one of each from their parents. Humans have 46 or 23 pairs. We number the chromosomes 1 to 23.  We have a # 1 from mom, a # 1 from dad, #2 from mom,#2 from dad, etc.The chromosome of each pair are called HOMOLOGOUS CHROMOSOMES

 Cells that have the pairs of their chromosomes like body cells are called DIPLOID  Diploid cells are said to be 2N where N= 23 in humans.  Cells that have only 1 of each chromosome like gametes are called HAPLOID  Haploid cells are 1N  So in humans our N =23, peas (7), Fruit fly(4 )

 So how do we get a cell that has the correct number of chromosomes to be the gamete or sex cell ( haploid cell)?  MEIOSIS- produces gametes that are 1N  = 46 sperm + egg = zygote (offspring)

Meiosis  Process of making gametes, sex cells 0r egg/sperm Humans have 46 chromosomes(23 pairs) that we get from our parents. Each pair is called a homologous pair.

Remember  Diploid cells contain 2 of each chromosome 2N- body cells 2N- body cells  Haploid cells contain 1 of each chromosome 1N- sex cell 1N- sex cell  N = # of pairs of chromosomes

2 Stages of Meiosis Meiosis I Meiosis II Diploid cell 4 Haploid cells Diploid cell 4 Haploid cells 2N 4 1N 2N 4 1N copy split copy split

Interphase  Chromosomes replicate 46 copy copy 92

Prophase I  Dna coils  Spindle fibers form  Homologous chromosomes pair up with their sister chromatid  All 4 together called TETRAD

TETRAD  Ends of chromosomes are sticky  This is where some of the genetic material exchanges places on its homologue.  This is how we get genetic variation.  CROSSING OVER

Metaphase I  Centromere attaches to the spindle fibers  Tetrads line up in the middle

Anaphase I  Homologous  pairs split

Telophase I  Spindle breaks up  Pairs at opposite ends  Cell splits  End up with cell with 46 chromosomes  Not done yet- split again to get to magic #23!

Meiosis II-2 nd division  Prophase II- spindle reforms  Metaphse II- line up in middle  Anaphase II- move apart  Telophase II – 2 nuclei at opposite ends  4 Haploid cells at end of Meiosis II- each has 1 copy of each chromosome.

How do we all look so different?  Crossing over  2 23 = 70 trillion different combinations  Nondisjunction- homologous pairs fail to separate during prophase I. One cell will have an extra chromosome. Ex. Down’s Syndrome has an extra #21 chromosome.  Gamete can have 1 less- Ex. Turner’s Syndrome has only 1 X chromosome instead of 2