Genetics The Mode of Inheritance DNA → chromosomes → genes → meiosis → gametes DEOXYRIBONUCLEIC ACID... DNA DNA is made of repeating segments. The combination of bases creates the genetic code. The bases are: Adenine A Thymine T Cytosine C Guanine G The two sides of the DNA are held together by the bases, where A only attaches to T and C only attaches to G DNA activity Remember that A goes with T and C goes with G Small differences in the pattern lead to GENETIC DIVERSITY ***diversity within species*** Big differences in the pattern or number of pairs lead to SPECIES DIVERSITY ***diversity between species*** Chromosomes Genes- one chromosome has MANY genes on it. Humans have 46 chromosomes... DNA...The Book of You Comparing chromosome numbers How many genes do other things have What do they ALL have in common? All genes are made of the same 4 bases, just in different combinations and numbers of each Cell Division-MITOSIS You start life as a zygote. Somehow every cell in your body since then has 46 chromosomes. How does that happen? Notice in this diagram that each of the 4 cells has a nucleus and inside each nuclei is EXACTLY the same number of chromosomes. Somehow these cells can CLONE themselves. This process is called mitosis One cell can become two IDENTICAL cells through the process of mitosis. The number of chromosomes in each cell does not change and the process can be repeated over and over again. The sequence of events in mitosis means that all of the chromosomes from the “mother” cell are copied and each “daughter” cell gets an exact copy Gamete Formation- MEIOSIS The sequence of events in meiosis means that each pair of chromosomes is separated from each other and then each chromosome is separated into sister chromosomes so that four GAMETES are formed. One DIPLOID (2n) mother cell divides to make four HAPLOID (n) gametes. Patterns of Inheritance Mendel’s Pea Plants Dominant and Recessive ALLELES When Mendel crossed a white flower with a violet flower, what did he expect to see? Dominant and Recessive ALLELES What he actually got was all white flowers. When he crossed THOSE white flowers, what did he expect to see? Dominant and Recessive ALLELES What he actually got was ¾ white flowers and ¼ violet flowers Why? There are TWO ALLELES for flower colorWhite and Violet. Mendel showed that white is DOMINANT over violet. This means that whenever the zygote has the allele for white flowers, the flower is white. When there is no allele for white flowers in the zygote, the flower will be violet. Purebred vs Hybrid The original parents were what we would describe as PUREBRED (aka homozygous): A white flower crossed with a white flower only produces white flowers; purple flowers crossed with purple flowers only produces purple flowers. In order to produce offspring in the ratio to the right, we crossed what would be called a HYBRID (aka heterozygous): The offspring that results from the cross between two different purebred parents. Here it would be a purebred white crossed with a purebred purple creates a hybrid white flower. Purebred vs Hybrid (homozygous vs heterozygous) We can use symbols to simplify the previous definition. “W” is the gene for flower color W is the allele for white flower color w is the allele for purple flower color Each parent has a PAIR of genes for flower color (one from it’s mom and one from its dad) before it makes gametes. That pair is then separated during MEIOSIS. Purebred vs Hybrid (homozygous vs heterozygous) “W” is the gene for flower color W is the allele for white flower color w is the allele for purple flower color A purebred white flower would have WW as it’s pair of genes before meiosis A purebred purple flower would have ww as it’s pair of genes before meiosis WW ww Purebred vs Hybrid (homozygous vs heterozygous) “W” is the gene for flower color W is the allele for white flower color w is the allele for purple flower color The white flower will give each gamete formed a W The purple flower will give each gamete formed a w WW ww Remember that every person has 2 alleles for every gene. After meiosis, one allele from each parent is passed on to the gamete. Here is some vocabulary: Purebred Hybrid (heterozygous)- (homozygous)- has two of has two different alleles (AKA heterozygous) the same alleles (AKA homozygous) Dominant - An allele that can mask the recessive trait every time Genotype - what alleles are present in the zygote Brown alleles are dominant over blue alleles in eye colour. We would assign B to represent this allele AA Aa aa Recessive - a trait seen if the dominant allele is NOT present. We would assign b to represent this allele Phenotype - what the zygote looks like Blue eyes are only possible when two copies of the recessive allele is present Brown eyes Blue eyes Make predictions about each of the parents below, using this chart as a key for the symbols to use Parent 1 - Purebred DominantPut the symbols for the genotype of this parent in the space below Parent 2 - Hybrid- Put the symbols for the genotype of this parent in the space below Hair Colour “B” Brown B Blonde b What two gametes can be formed from this genotype? Put the symbols for these in the space below and also at the top of the table on the next page What two gametes can be formed from this genotype? Put the symbols for these in the space below and also on the right side of the table on the next page Punnett squares help us predict the probability of what the offspring genotype and phenotype would be Offspring Genotypes Alleles in the gametes of parents Offspring Phenotypes To learn about how to use the Punnett Square, watch this video Let's try two HYBRID parents Parent one genotype Gametes produced Parent two genotype Gametes produced Offspring Genotypes Alleles in the gametes of parents Offspring Phenotypes What if a purebred blonde produced offspring with a hybrid mate? What are the chances of having a blonde baby? Parent one genotype Gametes produced Parent two genotype Gametes produced Offspring Genotypes Alleles in the gametes of parents Offspring Phenotypes Do Check and Reflect Questions pg 54 # 1, 2, 4, 5, 7 Thought Experiment: Answer the questions below Let’s take two species that we don't know and discover dominance and recessiveness. If we cross a purebred yellow bird with a purebred blue bird: How can we tell which is the dominant colour? What can we expect to see in the offspring? What would we see if two of the offspring were crossed? Patterns of Inheritance (continued) Where is DNA located, and what shape does it take? DNA is in the nucleus It is in a double helix (twisted ladder) What does dominant mean? A dominant allele is one that will show in either the homozygous or heterozygous combination What does recessive mean? Recessive alleles will only show when in the homozygous combination. Incomplete Dominance In this case, when the alleles are heterozygous, the phenotype is a blending of the traits. Make predictions about each of the parents, using this chart as a key for the symbols to use Flower Colour “R” Red R White r Alleles in the gametes of parents Offspring Genotypes Offspring Phenotypes Let's try two HYBRID parents Parent one genotype Gametes produced Parent two genotype Gametes produced Alleles in the gametes of parents Offspring Genotypes Offspring Phenotypes What if a pure bred white plant (homozygous recessive) produced offspring with a hybrid plant? What are the chances of having a red flowers? Parent one genotype Gametes produced Parent two genotype Gametes produced Alleles in the gametes of parents Offspring Genotypes Do Review Questions pg 55 # 1 to 9 Offspring Phenotypes Co-Dominance In this case, when the alleles are heterozygous, the phenotype shows BOTH of the traits. Let's try a purebred white cow with a purebred red bull Parent one genotype Gametes produced Parent two genotype Gametes produced Alleles in the gametes of parents Offspring Genotypes Offspring Phenotypes Let's try two HYBRID parents Parent one genotype Gametes produced Parent two genotype Gametes produced Alleles in the gametes of parents Offspring Genotypes Offspring Phenotypes Thought Experiment Lets consider a case of incomplete dominance. Welcome to the new baby, Orange Bird. If we were to look at the colour of the bird, who could his parents have been? If we were to look at the skill of this bird, who could his parents have been? Summary: There are 3 patterns of inheritance for heritable traits that result from SEXUAL reproduction 1. Complete dominance 2. Incomplete dominance 3. Co-dominance These, along with the formation of gametes lead to variation WITHIN a species.
