Understanding Mendelian Genetics: Traits, Alleles, and Genetic Inheritance
This guide explores the concepts of Mendelian genetics, focusing on how traits and characteristics are passed from one generation to another through genes located on homologous chromosomes. Key terms such as phenotype, genotype, alleles, dominant and recessive traits are defined, accompanied by examples including eye color and hair texture. The principle of dominance, law of segregation, and monohybrid crosses are explained, along with methods like Punnett squares for predicting genetic outcomes. Engaging activities encourage personal exploration of inherited traits.
Understanding Mendelian Genetics: Traits, Alleles, and Genetic Inheritance
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Presentation Transcript
Inheritance - Mendel • the transfer of characteristics (traits) from one generation to another • your inherited characteristics (traits) are determined by the genes located on your chromosomes
Gene • a short segment of a chromosome coding for one trait
Homologous chromosomes: • chromosomes are found in pairs • in humans, there are 46 chromosomes, made up of 23 pairs of homologous chromosomes • a pair of chromosomes consists of two homologous chromosomes which look alike and carry genes for the same traits • you receive one homologous chromosome of the pair from each parent, therefore you receive 2 genes for each trait
Phenotype • This term is used to describe the physical or visible appearance of an individual as determined by the gene combination you inherit from your parents. • Example: in the case of eye colour, having brown eyes is the phenotype and is dominant
Genotype • the gene combination that produces the trait • these genes may be the same or different • genotypes are represented by upper and/or lower case letters e.g. Bb • there are three possibilities for genotypes for a single trait. • e.g. BB or Bb or bb
Allele • alternate forms of a gene • same location on a each chromosome of pair • affects the same trait but differently • alleles are different forms of a gene that carry different instructions • eg. Brown eyes or blue eyes
Dominant Trait/Allele • Characteristic that is always expressed its gene is present • only one dominant allele needs to be present for the dominant trait to be expressed • an upper case letter designates a dominant allele • eg. Tall is dominant. It is represented with a “T”
Recessive Trait/Allele • Characteristic that is only expressed when two genes for that trait are present • two recessive alleles need to be present for a recessive trait to be expressed • if a dominant allele is present it “masks” the recessive allele • eg: Short is recessive. It is represented with a • “t”
Homozygous Genotype • “homo” means the “same” • the condition where both alleles for a trait are the same • there are two conditions for homozygous genotype: • homozygous recessive genotype “aa” • homozygous dominant genotype “AA”
Heterozygous Genotype • “hetero” means “different” • the condition where both alleles for a trait are different • the genetic information inherited for a trait from both parents is different • example: heterozygous genotype “Aa”
Activity • Introduction to Variability • Worksheet • Compile your own personal profile, and then combine it with the other members in the group. • Ratio for group • Ratio for class
Mendel’s Experiment • Crossed two purebred parents (One tall and one short) • All offspring were tall!! Mendel concluded that some traits were ___________ and some traits were ___________.
Principal of dominance • When individuals with contrasting traits are crossed, the offspring (F1 generation) will express only the dominant trait. Parents: Tall x Short F1 (Offspring): Tall Tall Tall Tall
What happened next?? • Mendel crossed F1 generation. • F1: Tall plants x Tall plants • F2 (Offspring of F1): Tall Tall Tall Short • F2 generation = 3:1 ratio
Law of Segregation • Each F1 parent starts with two hereditary factors (alleles); one is dominant and one is recessive • Each parent contributes only one factor (allele) • Each offspring inherits one factor (allele) from each parent • If the dominant factor (allele) is present it will be expressed. • If the recessive factor (allele) is present it will only be expressed if only recessive factors are present.
Monohybrid Cross • Mendel crossed two purebred plants • TT – tall plant • Tt – short plant • Monohybrid cross – only one trait is being tested
PunnettSquares • Punnett Square • Is a grid system resembling a checkerboard, used in computing possible results of various genetic combinations • Simply stated, it is a way of representing the possible combinations of genes when an egg and sperm unite in fertilization
Predicting genotype and phenotype possibilities using Punnett Squares Characteristic hair texture – • Dominant allele (gene) is curly hair - C. • Recessive allele (gene) is straight hair - c • Mother • Phenotype – straight hair • Genotype- homozygous recessive - cc • Father • Phenotype – curly hair • Genotype – homozygous dominant – CC
Punnett Square All of their children will have curly hair. They all have a different genotype from their parents.
Homework…. Bikini Bottom Genetics – Monohybrid Crosses Due tomorrow (Wednesday) – Will be marked and recorded.
What happened when Mendel crossed two pea plants that different in TWO traits? In his second experiment Mendel crossed a pea plant with round/yellow (RRYY) seeds with a pea plant with wrinkled/greed seed (rryy). What genotypes resulted? What phenotypes resulted? Genotype: RrYy F1 - Generation Ratio 100% Phenotype: Round/Yellow Ratio 100%
Crossing F1(RrYy) x F1(RrYy) Genotype: RRYY RRYy Rryy RrYY RrYy rrYY rrYy rryy Ratio: 1 2 2 2 4 1 2 1 Phenotype: Round/Yellow Round/Green Wrinkled/Yellow Wrinkled/Green Ratio: 9 3 3 1 F2 - Generation What genotypes resulted? What phenotypes resulted?
Law of Independent Assortment • The inheritance of alleles for one trait ________ (does/does not) affect the inheritance of alleles for another trait.
