Genetics (10.2, 10.3, Ch.11) SB2. Students will analyze how biological traits are passed on to successive generations. (c) Using Mendel’s laws, explain the role of meiosis in reproductive variability. (e) Compare the advantages of sexual reproduction and asexual reproduction in different situations.

Some Needed Vocabulary Heredity Passing on of characteristics from parent to offspring Traits Characteristics that are inherited Genetics The study of Heredity Gametes Sex cells Sperm = male gamete Egg = female gamete

Gregor Mendel A monk who studied heredity using garden pea plants Why garden peas? They reproduce sexually They have both male & female gametes on the same plant What did he do? He transferred pollen from one plant to another with different traits This is called “making a cross” Mendel first “crossed” tall plants with short plants

Mendel’s Laws Law of segregation The two alleles for each trait must separate when gametes are formed A parent will pass down AT RANDOM, only one allele for each trait Law of independent assortment Genes for different traits are inherited independently of one another

Mendel’s Monohybrid crosses Monohybrid cross Focuses on one (mono) trait 1 st generation Crossed a tall pea plant with a short pea plant All of the offspring were tall

Mendel’s Monohybrid crosses 2 nd generation Crossing the offspring of the 1 st generation ¾ of the offspring were tall; ¼ of the offspring were short 3:1 ratio (tall to short)

Mendel’s Findings Mendel found that each chromosome has two factors for each traits… called alleles Alleles = forms of genes Types of alleles Dominant The allele that has the ability to mask the other Represented with a capital letter (A) Recessive The allele that will be masked by the dominant allele… it will be there, but will not be expressed Represented with a lowercase letter (a)

Phenotype vs. Genotype Phenotype The way that an offspring looks; how the trait appears Tall, short Genotype The offspring’s genetic combination TT, Tt, tt

Homozygous vs. Heterozygous Homozygous (“homo” means “same”) Having two identical alleles for a trait TT, tt Heterozygous (“hetero” means “different”) Having two different alleles for a trait Tt

Punnett square Short way to find the expected proportions of possible genotypes in the offspring of a testcross

Monohybrid cross vs. dihybrid cross Monohybrid cross Involves the cross of ONE trait Uses a 4 block punnett square Dihybrid cross Involves the cross of TWO traits Uses a 16 block punnett square “F.O.I.L.” to produce gametes

Complete Dominance Inheritance characterized by an allele that is fully expressed in the phenotype of a heterozygote and that masks the phenotypic expression of the recessive allele Example: In humans, dimples are dominant over no dimples. What is the probability of producing a dimpled child from two heterozygous dimpled parents?

Exceptions to Mendel’s Law Mendel’s traits showed two distinct forms Most genes do not exhibit simple inheritance Genotypic ratios persist but phenotypic ratios may vary because of interactions between Alleles Other genes Segregation of genes on the same chromosome Environment

Lethal Alleles Some allele combinations are lethal Must be taken into consideration when completing punnett squares

Incomplete Dominance Pattern of inheritance in which the dominant phenotype is not fully expressed in the heterozygote, resulting in a phenotype intermediate between the homozygote dominant and homozygous recessive Example: In snapdragons (a type of plant), pink and white petal colors show incomplete dominance. What is the likelihood of producing a red flower if two pink flowers are crossed?

Codominance Inheritance characterized by full expression of BOTH alleles in the heterozygote Example: In birds, specifically chickens, black and white feather color is expressed in a codominant fashion. What is the likelihood of producing a black bird if a white bird and black bird are crossed?

Multiple Alleles Some genes may have more than just two alternative forms of a gene The inheritance of the ABO blood groups is an example of a locus with three alleles

Multiple Alleles Example: In humans, one of the ways blood types are determined is with a trait controlled by multiple alleles. What is the likelihood of producing a child with type O blood from two heterozygous parents one with type A blood and one with type B blood?

Interaction between two non-allelic genes in which one modifies the phenotypic expression of another In horses, brown coat color (B) is dominant over tan (b). Gene expression is dependent on a second gene that controls the deposition of pigment in hair. The dominant gene (C) codes for the presence of pigment in hair, whereas the recessive gene (c) codes for the absence of pigment.Epistasis

Polygenic Inheritance Mode of inheritance in which the additive effect of two or more genes determines a single phenotypic character

Sex-Linked Alleles Controlled by genes located on sex chromosomes (XX or XY) Usually carried on X chromosome Since females are XX, they are usually carriers of the trait Since males are XY, they have a higher tendency for inheritance and EXPRESSION of the trait Example: In humans, there is a genetic condition known as Hemophilia that is controlled by sex-linked inheritance. What is the likelihood of having a female carrier if a hemophiliac dad and a normal mom are crossed?