Patterns of Inheritance

• Sex-limited traits are those where only one sex can express the phenotype, such as plumage in chickens and facial hair in lions.
• Sex-influenced traits are those where sex determines how a phenotype is seen, meaning male and female individuals, which are genotypically similar for a particular trait, give different expressions of the same trait, such as horns in sheep.
• In sex-influenced traits, females may not express the phenotype as strongly as males, for example, BB in females is not as pronounced as that of males.
• Plant Part: Seed Shape, Color, Pod Shape, Flower Color, and Flower Position are all contrasting traits.
• Crosses can occur between two homozygotes, between a homozygote and a heterozygote, or between two heterozygotes.
• F1 results indicate that all round seeds have a 2.96:1 ratio of round to wrinkled seeds, all yellow flowers have a 3.01:1 ratio of yellow to green flowers, all full pods have a 2.95:1 ratio of full to constricted pods, all violet flowers have a 3.15:1 ratio of violet to white flowers, and all axial flowers have a 3.14:1 ratio of axial to terminal flowers.
• If the problem specifies that the man’s mother has blood type O, it provides a clue that the man is heterozygous.
• Mendelian laws state that each individual has a pair of factors (alleles) for each trait, and these factors (alleles) segregate/separate during gamete formation.
• Polygenic inheritance is when one character is influenced by many genes, with an additive effect of two or more genes on a single phenotypic character.
• In a monohybrid cross, only one type of gamete can be formed: T for the tall parent and t for the short parent.
• Quantitative characteristics exhibit a normal distribution, also known as a bell-shaped curve.
• Examples of polygenic inheritance in humans include height, skin color, and eye color.
• When two F1 individuals are crossed, each of them can contribute 2 types of gametes since segregation of the alleles will give us T and t.
• Sex-related inheritance involves sex linkage, where the phenotypic expression of an allele is related to the chromosomal sex of the individual.
• In a monohybrid cross, homozygous individuals can contribute only 1 types of gamete, while heterozygous individuals can contribute 2 types of gametes.
• A Punnett Square is a table listing all possible genotypes resulting from a cross, with all possible sperm genotypes lined up on one side and all possible egg genotypes lined up on the other side.
• Sex-linked traits are those that are determined by genes on one of the sex chromosomes.
• To make a Punnett Square, find the genotype of parents (if it is not given in the problem) and choose a letter to represent the trait (uppercase version of the letter for dominant allele and lowercase version of the same letter to represent recessive allele).
• Draw the Punnett square, putting the female parent on the left and the male parent at the top.
• X-linked traits are those where the gene that encodes for the trait is located on the X chromosome.
• Y-linked traits are those where the gene that encodes for the trait is located on the Y chromosome.
• X-linked dominant traits include some forms of retinitis pigmentosa, Chondrodysplasia Punctata, hypophosphatemic rickets, and others.
• X-linked recessive traits include Duchenne muscular dystrophy, hemophilia A, X-linked severe combined immune disorder (SCID), red-green color blindness, and some forms of congenital deafness.
• Sex influences phenotype in sex-limited inheritance and sex-influenced inheritance.
• Autosomal genes are involved in sex-limited inheritance.
• Hormones are involved in sex-influenced inheritance.
• Dihybrid cross starting with a parent which is homozgous dominant for 2 characters and another parent which is homozygous recessive for 2 characters generated F1 which are heterozygous for both characters.
• On crossing F1 with each other, 16 F2 individuals are expected at a 9:3:3:1 phenotypic ratio.
• There are a lot more genotypes expected now and only 1 of the 16 showing the homozygous dominant trait and another showing the homozygous recessive trait.
• From these ratios we can predict the probability of the occurrence of a phenotype or genotype.
• Based on the 3:1 F2 ratio of the monohybrid cross, dominant phenotype has 75% probability, while the recessive phenotype has a 25% probability.
• The values can also be expressed as fraction.
• In a cross between an individual which is heterozygous for 2 characters and an individual who is homozygous recessive for both characters, the probability of each of the following offspring can be determined: homozygous recessive, phenotypically dominant for both characters, phenotypically recessive for both characters, and heterozygous for both characters.
• If more than 3 characters are involved, and the problem does not specify or require the use of a Punnett square, there is a shorter way of solving for phenotypic or genotypic ratio/probabilities.
• The probability of having an offspring with the genotype AaBbCcDdEE can be determined by plugging in values from the results of the crosses in the previous slide.
• The probability of having an offspring with the phenotype ABCdE can be determined by determining the phenotypes resulting from the genotypes and plugging in their values.
• The probability of having an offspring with the genotype AABbCCddEE can be determined by plugging in values from the results of the crosses in the previous slide.
• The probability of having an offspring with the genotype AaBBccDdEe can be determined by plugging in values from the results of the crosses in the previous slide.
• The probability of having an offspring with the phenotype abcde can be determined by determining the phenotypes resulting from the genotypes and plugging in their values.
• The probability of having an offspring with the phenotype ABCDE can be determined by determining the phenotypes resulting from the genotypes and plugging in their values.