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Cytogenetics Lesson 13: Heredity Terms and Mendelian Laws of Inheritance, Lecture notes of Cell Biology

Silliman University (SU)Cell Biology

The basic terms and concepts related to heredity, including alleles, genotype, phenotype, and Mendelian laws of inheritance. It also explains the ways to track heredity, such as Punnett squares and pedigrees. The chromosomal basis of inheritance is also discussed, including allele segregation during meiosis.

Typology: Lecture notes

2022/2023

Available from 01/23/2024

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CYTOGENETICS
Lesson
13
[TRANS] LESSON 13: HEREDITY
TERMS IN HEREDITY
ALLELES
●Alleles
○ Alternative versions of a gene
GENOTYPE
● Genotype
○ Whole collection of alleles possessed by
an individual
PHENOTYPE
● Phenotype
○ Observable characteristics of an
individual
PREVIOUS HYPOTHESIS ON HEREDITY
● Offspring were copies of only one parent
●“One disproven theory of inheritance suggested
that genetic traits are passed down solely from
the father. In support of this particular theory of
uniparental inheritance, some early microscopists
fancied they could see a small, perfectly formed
human crouched inside the head of each sperm.”
GREGOR MENDEL
●Used true-breeding plants
●“True-breeding varieties, when cross-fertilized
with each other, produce hybrid offspring that
resemble one parent. The appearance of the
F2generation shows that an individual carries two
alleles of each gene.”
●Offspring inherit one copy of a gene each from
the mother and the father
● Homozygous
○ 2 identical alleles
● Heterozygous
○ 2 distinct alleles
● Gregor Mendel examined various inheritable
traits in pea plants
●“Mendel studied seven traits that are inherited in a
discrete fashion. For each trait, the plants display
either one variation or the other, with nothing in-
between. As we will see shortly, one form of each trait
is dominant, whereas the other is recessive.”
MENDELIAN LAWS OF HEREDITY
LAW OF SEGREGATION
●2 alleles for each trait separate during gamete
formation
● unite at random-one from each parent—at
fertilization
● Applies to all sexually reproducing organisms
○ 3:1 ratio
●“Mendel’s law of segregation explained the data for
every trait he exam- ined in pea plants, and he
replicated his basic findings with corn and beans. But
his rules governing inheritance are not limited to
plants: they apply to all sexually reproducing
organisms .”
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Download Cytogenetics Lesson 13: Heredity Terms and Mendelian Laws of Inheritance and more Lecture notes Cell Biology in PDF only on Docsity!

CYTOGENETICS

Lesson

[TRANS] LESSON 13: HEREDITY

TERMS IN HEREDITY

ALLELES

● Alleles ○ Alternative versions of a gene GENOTYPE ● Genotype ○ Whole collection of alleles possessed by an individual PHENOTYPE ● Phenotype ○ Observable characteristics of an individual PREVIOUS HYPOTHESIS ON HEREDITY ● Offspring were copies of only one parent ● “ One disproven theory of inheritance suggested that genetic traits are passed down solely from the father. In support of this particular theory of uniparental inheritance, some early microscopists fancied they could see a small, perfectly formed human crouched inside the head of each sperm.” GREGOR MENDEL ● Used true-breeding plants ● “T rue-breeding varieties, when cross-fertilized with each other, produce hybrid offspring that resemble one parent. The appearance of the F 2 generation shows that an individual carries two alleles of each gene.” ● Offspring inherit one copy of a gene each from the mother and the father ● Homozygous ○ 2 identical alleles ● Heterozygous ○ 2 distinct alleles ● Gregor Mendel examined various inheritable traits in pea plants ● “ Mendel studied seven traits that are inherited in a discrete fashion. For each trait, the plants display either one variation or the other, with nothing in- between. As we will see shortly, one form of each trait is dominant, whereas the other is recessive.” MENDELIAN LAWS OF HEREDITY LAW OF SEGREGATION ● 2 alleles for each trait separate during gamete formation ● unite at random-one from each parent—at fertilization ● Applies to all sexually reproducing organisms ○ 3:1 ratio ● “Mendel’s law of segregation explained the data for every trait he exam- ined in pea plants, and he replicated his basic findings with corn and beans. But his rules governing inheritance are not limited to plants: they apply to all sexually reproducing organisms .”

LAW OF DOMINANCE AND UNIFORMITY

● Dominant alleles ○ Dictate the phenotype when they are present ○ Only 1 copy needed ● Recessive (hidden) alleles ○ Dictate the phenotype in the absence of a dominant allele ○ 2 copies needed LAW OF INDEPENDENT ASSORTMENT ● Alleles of different genes segregate independently of one another ● Characteristic 9:3:3:1 phenotypic ratio WAYS TO TRACK HEREDITY PUNNETT SQUARE ● A square diagram that is used to predict the genotypes of a particular cross or breeding experiment. ● “The gray box at the bottom, called a Punnett square after a British mathematician who was a follower of Mendel, allows one to track the segregation of alleles during gamete formation and to predict the outcomes of breeding experiments like the one outlined in Figure 19−22. According to the system established by Mendel, capital letters indicate a dominant allele and lowercase letters a recessive allele.” PEDIGREES ● Displays the phenotype of each family member for the relevant trait

● Geneticists that follow the inheritance of specific

traits in humans get around this problem by working with large numbers of families—or with several generations of a few large families—and preparing pedigrees that show the phenotype of each family member for the relevant trait. ● “ A pedigree shows the risks of first-cousin marriages. Shown here is an actual pedigree for a family that harbors a rare recessive mutation causing deafness. According to convention, squares represent males, circles are females. Here, family members that show the deaf phenotype are indicated by a blue symbol, those that do not by a gray symbol. A black horizontal line connecting a male and female represents a mating between unrelated individuals, and a red horizontal line represents a mating between blood relatives. The offspring of each mating are shown underneath, in order of their birth from left to right.” CHROMOSOMAL BASIS OF INHERITANCE ALLELE SEGREGATION ● Caused by the separation of chromatids during meiosis ● “ The separation of duplicated homologous chromosomes during meiosis explains Mendel’s laws of segregation and independent assortment. Here we show independent assortment of the alleles for seed color, yellow (Y) and green (y), and for seed shape, round (R) and wrinkled (r), as an example of how two genes on different chromosomes segregate independently. Although crossovers are not shown, they would not affect the independent assortment of these traits, as the two genes lie on different chromosomes.”

● The most common human disorders are due to many genes acting together; DNA sequencing studies are identifying mutations in these genes that increase the risk of developing these diseases.