Concepts Covered 1. Sexually reproducing organisms make gametes (sex cells). gametes (sex cells). 2. Genes are stable, control traits, and are passed on in gametes. on in gametes. 3. Genes are part of chromosomes which are found in pairs in somatic cells. found in pairs in somatic cells. 4. Alleles are alternative versions of genes 5. Organisms can be heterozygous or homozygous 6. Gene pairs associate and then separate during gamete formation so that each gamete gets gamete formation so that each gamete gets one gene from the pair one gene from the pair
7. Gametes combine at random to form the individuals in the next generation individuals in the next generation 8. One allele can be dominant over another or show lack of dominance show lack of dominance 9. The segregation of genes allows geneticists to make predictions using Punnett squares make predictions using Punnett squares 10.The segregation of different gene pairs controlling different traits can be predicted based on different traits can be predicted based on their independent assortment. their independent assortment. Concepts Covered
Objectives - Define the terms: gene, chromosome, gamete, somatic, genotype, phenotype, alleles, homozygous, heterozygous, segregation, independent assortment, Punnett square, F1 generation - Review / become familiar with: - the process of sexual reproduction in plants - gamete formation (meiosis) - predicting possible gametes from parents - predict possible offspring using a Punnett square Punnett square
The majority of cells in a plant are somatic cells. Somatic cells have 2 copies of each chromosome. Thus, they also have 2 copies of every gene. Plants also have gamete cells (sex cells). They have only 1 copy of each chromosome giving them half the number of chromosomes in a somatic cell. This also means they have only 1 copy of every gene.
Genes on a chromosome are often depicted with letters. ‘A’ could represent the Bt gene and labels only one gene of thousands on that chromosome. A A Gene Pair
Sometimes the two genes in a chromosome pair are the same. These chromosomes are homozygous for that particular gene. These genes can be labeled with the same letter and letter case showing they are the same version of the same gene. A A
Sometimes the two genes in a chromosome pair are different versions of the same gene. These versions are called alleles of that gene. These chromosomes are heterozygous for that particular gene. The different alleles can be labeled with lower and upper case letters. A A b B The allelic make up of an organism is its genotype.
Pollen (sperm in animals) Egg Gametes (sex cells) Sexual Reproduction in Plants
Egg Hereditary Material (DNA stored as chromosomes) Sexual Reproduction in Plants Pollen (sperm in animals)
Egg A pollen tube grows from the pollen grain into the egg. Sexual Reproduction in Plants Pollen (sperm in animals)
Egg The pollen DNA is delivered into the egg via the pollen tube. Sexual Reproduction in Plants Pollen (sperm in animals)
Zygote (single cell) Nucleus Sexual Reproduction in Plants
Zygote (single cell) Nucleus Sexual Reproduction in Plants Has 2 copies of every gene.
Chromosomes Half of the chromosomes in the nucleus came from the male parent and the other half from the female parent.
The single cell divides. All of the genetic information in the first cell is replicated and passed on to the other cells. All cells will have a copy of all of the genetic material (DNA).
The cells continue to divide and differentiate into parts of the plant.
The process begins all over again. Pollen Egg (silk) Gametes (sex cells) HereditaryMaterial(DNA)
A A Egg (silk) HereditaryParticle(Gene) a A gene is a small segment of DNA on a chromosome. A gene encodes a single protein. Proteins control traits. Pollen
AA Pollen Egg There are thousands genes in each gamete. We will follow 1 gene through the process of sexual reproduction. Sexual Reproduction in Plants a
Zygote (single cell) Nucleus Sexual Reproduction in Plants Gene Pair Aa
AaAa Aa Aa Aa Aa Aa Aa Aa Somatic (body) cells have two copies of every gene. 12
AaAa Aa Aa Aa Aa Aa Aa During meiosis gametes (sex) cells are formed. In meiosis, paired genes separate so each gamete has only one copy of every gene. This is called segregation. Male Parent AA aa Pollen View the process of meiosis.
To simplify things, we’ll look at only 4 chromosomes and follow a single gene. First, the chromosomes replicate. When this happens, a copy is made of the DNA in each chromosome. At this time, there are actually 4 copies of each gene in the cell. Gamete Formation b B B B b b
The replicated chromosome pairs line up in the middle of a cell and segregate into two separate cells. Chromosomes segregate randomly. When chromosomes pair and separate during gamete formation, a chromosome could line up on either side of the pair and therefore segregate into either one of the daughter cells. As a result, there are many possible chromosome combinations in the daughter cells. B Bb b B b B b
The replicated chromosome pairs line up in the middle of a cell. Spindle fibers within the cell pull the pairs apart. A new cell wall forms around each single set of chromosomes. B b B b B b B b B Bb b
Within these two newly formed cells, the single chromosomes line up in the middle of the cell. Each replicated chromosome splits and spindle fibers pull the two halves apart. B b B b B b B b
A new cell wall forms around the chromosomes. The result is four gametes with only one copy of each chromosome. B b B b B b B b
AaAa Aa Aa Aa Aa Aa Aa A) Only gametes with the same alleles come together B) Only gametes with different alleles come together C) Randomly AA aa Pollen Male Parent Aa Aa Aa Aa Aa Aa Female Parent Eggs aA QUESTION? How do gametes come together?
AaAa Aa Aa Aa Aa Aa Aa AA aa Male Parent Aa Aa Aa Aa Aa Aa Female Parent aA
AA aa a A AAA A a aaa ANSWER! Gametes come together randomly. As a result, there are multiple gene combinations possible in the progeny.
The combination of genes in a cell or organism is its genotype. Genes encode proteins. Proteins control traits. Genotype vs. Phenotype Traits are observable characteristics which make up thephenotype. which make up the phenotype.
Phenotype The phenotype of a plant is its observable characteristics (traits). Examples would be plant height and formation of disease-like lesions. Phenotype is the product of the genotype of the plant and the environment in which the plant is grown. Phenotype = Genotype X Environment
Dominance of Alleles Sometimes one allele will mask the expression of another. These are dominant alleles, while those that are masked are called recessive. AA (Normal) aa (Lesion) Aa (Normal) A = Dominant allele for normal leaves a = Recessive allele for disease-like lesions a = Recessive allele for disease-like lesions
Dominance of Alleles Sometimes two alleles lack dominance over one another. In this case, the offspring would have a phenotype that was unique or intermediate to either parent. AaaaAA Pale Green Pale GreenYellow Dark Green Genotype = Phenotype =
A) AA B) Aa C) aa D) white QUESTION? In corn, salmon colored silk is dominant to white. What would be the phenotype of a white silked corn plant?
Phenotype refers to the observable characteristic (traits) of a gene. In this case, the phenotype would be “D) white” and all other possible answers refer to genotypes. Phenotype refers to the observable characteristic (traits) of a gene. In this case, the phenotype would be “D) white” and all other possible answers refer to genotypes. ANSWER!
A) AA B) Aa C) aa D) white QUESTION? What would the genotype of the white silk corn plant be?
Answer! Since white silk is recessive to salmon colored silk, a white silk corn plant will only have gene alleles aa. Therefore, its genotype is “C) aa”. Since white silk is recessive to salmon colored silk, a white silk corn plant will only have gene alleles aa. Therefore, its genotype is “C) aa”.
Genes on the same chromosome assort independently of each other into the gametes during gamete formation (meiosis). As a result, there are many gene combinations possible in the gametes. Independent Assortment
Chromosome pairs could line up different ways line up different ways Parent Genotype = BbCc BBbb CCcc BB CC bb cc B C b c BBbb ccCC BB cc bb CC B c b C Replication Separation of chromosome pairs Chromosomes split into gametes B c B c b C b C B C B C b c b c
Parent Genotype = BbCc 4 Possible Gametes B c B c b C b C B C B C b c b c B c b C B C b c
These predictions are often made using Punnett squares. - Genotypes (allelic composition) of parents and offspring - Phenotypes (observable traits) of parents and offspring - The ratios of genotypes and phenotypes of offspring Predicting offspring genotypes with the Punnett Square. The segregation of genes allows geneticists to predict:
Punnett Square Example A plant breeder makes a cross between a male plant with the phenotype ‘AA’ (pink silk) and a female plant with the phenotype ‘aa’ (white silk). Assuming the ‘A’ allele is dominant over the ‘a’ allele, what would be the genotypes of the F1 generation and their ratio? AAaa X Pink Silk White Silk
aa AA aa A A The female parent will donate 2 ‘a’ gametes. The male parent will donate 2 ‘A’ gametes.
aa AA aa A A Fertilization occurs. The offspring is called the F1 generation. Aa
AaAaAaAa aa AA aa A A When the gametes come together, 100% will have the genotype ‘Aa’ and 100% will have the phenotype of pink silks. AaAaAaAa AaAaAaAa AaAaAaAa
Summary - Genes encode proteins that control traits - Different versions of a gene are called alleles - Genotype = allelic composition - Phenotype = traits (observable characteristics) - Heterozygous: different alleles in a gene pair (Aa) - Homozygous: same genes in a gene pair (AA, aa)
- Segregation: separation of paired genes giving each gamete one gene copy giving each gamete one gene copy - Independent assortment: genes assort into gametes independently of each other into gametes independently of each other - Gametes combine randomly during fertilization - Alleles can be dominant, recessive, or show lack of dominance lack of dominance - Punnett squares are used to predict offspring genotype, phenotype, and ratio genotype, phenotype, and ratio Summary