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Results and Conclusions of Mendel’s Experiments
Mendel began his experiments by cross pollinating pure breeding plants
• His studies focused on only one characteristic at a time for example, flower color. This type of cross is called a Monohybrid cross
Mendel developed a mathematical vocabulary to identify different generations of pea plants
• P1: Parent generation• F1: First (“filial”meaning offspring) generation
Offspring of P1 cross P1 x P1• F2: Second Generation - Offspring of F1 cross
All F1 plants are purple (Ww )…
Where did the WHITE form go?
3 out of 4 offspring with purple flowers
WW Ww Ww ww
Ww WwWw Ww
What is the resulting ratio of purple to white flowers in the F2 generation?
• Purple 3:1 white
Express this answer as a percent • 75% purple: 25 % white
Now let’s look at another…
1) Parents transmit information about traits to their offspring.
• Mendel called this information a “factor.” Today we know that “factors” are really genes.
2) Each parent must be contributing a form of a gene to the offspring.
• From our studies in the last unit, we know this is true because chromosomes came in pairs called homologous chromosomes. Each homolog contains the same genes, so a gene pair exists for each gene on the chromosome.
Gene PairGene locus
AlleleAlleleGTACCAGGA ATGCCGGGT
(Yellow) (Green)
2) Each parent must be contributing a form of a gene to the offspring.
• The two alleles in a gene pair can be:
1) identical or 2) different • For example, a gene pair which
codes for seed color in pea plants could contain:
1) yellow, yellow (purebred or2) green, green homozygous)3) yellow, green (hybrid) -
heterozygous
• Gene pairs are described in two ways:1) homozygous: alleles in gene pair are identical Pure breeding plants are homozygous.2) heterozygous: alleles in gene pair are differentHybrid plants are hererozygous.
3. Symbols can be used to represent alleles.
• If you use an uppercase “W” to represent purple as the dominant form of pea color and use a lowercase “w” to represent white as the recessive form of pea color, you can illustrate the allele combinations for the following pea plants. (WW, Ww, ww)
4. Genetic makeup and observed characteristics are not always the same!
GENOTYPE• the exact form of the alleles
in a gene pair• Example: WW, Ww, ww
PHENOTYPE• the observable or expressed
form of a trait resulting from a gene pair
• Example: yellow or green seed color
In pea plants, Green pod color is dominant over yellow.
Description GENOTYPE PHENOTYPE
Homozygous dominant GG Green
Homozygous recessive gg Yellow
Heterozygous Gg Green
Heterozygous recessive?? No such thing!
Mendel’s Laws
1. The law of Dominance• States that if two alleles in a gene pair are
different, then one allele can control the trait and the other one can be hidden.
• Example: Green pod color is seen in plants with the genotype Gg
Mendel’s Laws
2. The law of Independent Assortment• States that gene pairs segregate into gametes
randomly and independently of each other.• At what stage of meiosis does this occur?
Metaphase I and Anaphase I of meiosis-homologs randomly separate
Mendel’s Laws
3. The law of Segregation• States that each pair of genes segregates, or
separates, during meiosis and each gamete contains one gene from each gene pair.
• What is the chance that a gamete will end up with a maternal homolog at any one chromosome pair?
• ½ or 50%• The chance that all 23 homologs from one parent
will end up in a single gamete is 1 in 8 million (1/2)23
What is a monohybrid cross?
• A monohybrid cross is when a scientist studies the inheritance of one trait at a time.
How is the following statement “Each parent must be contributing a form of a gene to the
offspring” supported by what you know about meiosis and homologous chromosomes?
The chromosomes from each homologous pair come from the parents (one from each). Each homologous pair carries a pair of genes (one gene on each homolog). Therefore one copy of each gene comes from each parent.
What is the difference between homozygous and heterozygous?
• Homozygous means each gene has the same allele.
• Heterozygous means that each gene in the pair has different alleles.
What is the relationship between phenotype and genotype?
• Genotype is what alleles are found in a pair of genes. Phenotype is the outward appearance of those alleles.
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