Chapter 14: Genetics (C8478026)

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1 Mendelian concepts

Gregor Mendel was an Austrian friar who lived in the 1800’s, who demonstrated inheritance of certain traits in pea plants, from one generation to the next, known as pure breeding, where traits are passed on to all subsequent generations. Characteristic is an observable feature, and trait is the description of that feature. For example, eye color is a characteristic; and blue, brown or hazel is a trait. However, when Mendel crossbred plants with different traits, the result was not a blend, but one trait would appear in all offspring. This type of trait is known as the dominant trait. The offspring were the first generation, known as F1 generation, F1 an abbreviation for Filial 1. The parental generation is abbreviated P. However, when Mendel crossbred the F1 generation, only 75% of the F2 generation expressed the dominant trait; and 25% expressed the other trait, known as the recessive trait. Mendel thus theorized that genes thus had two alleles, an allele representing each of its trait. [In fact, there can be more than two alleles, for example, blood type which has three alleles, but only one allele can occupy the locus at a time.] The dominant allele is denoted with a capital letter, and the recessive allele is denoted with a lower case letter. In Mendel’s example, his two alleles were “A” denoting purple, and “a” denoting white. Genotype refers to the combination of alleles present, which therefore determines the traits that will show. The word “allele” therefore is used loosely to refer to the constituent alleles making up the genotype. If both alleles of the genotype are the same, the individual is known as homozygous for that trait, and the trait is expressed directly (as demonstrated in true breeding). Homozygous is denoted as either “AA” or “aa”. If the genotype has two different alleles, the individual is known as heterozygous for that trait, and only the trait of the dominant allele [and not the recessive allele] is expressed. Heterozygous is denoted as “Aa”, the capital letter in a genotype usually written before the smaller letter. Phenotype is the expression of the trait.

Mendel’s 1st Law (aka Law of Segregation) is that one allele [of the two alleles in the parent genotype], and only one, is randomly passed through the gamete. In other words, traits do not “mix”. This occurs, because alleles are found in the same locus, which is the location on the chromosome. Gametes randomly receive one [of the two] chromosomes, and therefore only receives one allele [randomly].

Punnett square can be used to predict the genotype of a breeding experiment. The alleles that can be present in the paternal gamete should be listed horizontally, and the alleles that can be present in the maternal gamete listed vertically. The gametes are then combined in the respective squares. Phenotypic ratio, which is the ratio of the genotypes to each other. In the example , assuming dominance, the phenotypic ratio is 3:1, as 75% will show the dominant trait, and 25% will show the recessive trait.

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Hybrid is the offspring resulting from the mating of two genetically unlike individuals. Monohybrid is the offspring resulting from the mating of two individuals that differ (have both a dominant and recessive) in one trait of interest. In contrast, dihybrid is the offspring resulting from the mating of two individuals that differ (have both a dominant and recessive) in two traits of interest. The phenotypic ratio of a dihybrid cross is 9:3:3:1. This means of every sixteen offspring, nine individuals will express the dominant trait of both characteristics, three individuals will be dominant for the first and recessive for the second, three other individuals dominant for the second and recessive for the first, and one individual will express the recessive trait of both characteristics. Note that dihybrid assumes that each allele is on a different chromosome (For more information, see Mendel’s 2nd Law).

Mendel’s 2nd Law (aka Law of Independent Assortment) is that alleles [and thus traits] are inherited independent of each other. In other words, just because you have one trait, doesn’t increase or decrease the likelihood of having another trait. This law is limited, because Mendel’s study was of alleles in different chromosomes. As stated , since random allocation of allele is due to gametes randomly receiving one [of the two] chromosomes, if alleles are on the same chromosome, allocation would not be random, either having an increased or decreased likelihood when crossover (described ) occurs.

Just as Mendel’s 2nd Law is limited, so is Mendel’s 1st Law. One allele does not always express dominance. The following genetic interactions influence phenotype:

• Incomplete dominance (aka partial dominance), where the phenotype (what shows) of the heterozygous (Aa) genotype is a blend of the phenotype of the homozygous (AA or aa). For example, a pink flower that results from crossing a (homozygous) red flower with a (homozygous) white flower.

“You know how Kate Hudson photoshopped her picture with Matthew McConaughey, it’s so cute!” Mandy said, “Let’s do this for you and Sophie, but the science way!”

“So Jamie’s black hair is represented as BB, and Sophie’s blonde hair as bb,” Blaire continued, “the Punnett Square providing Bb, Bb, Bb, and Bb.”

“Hair color is incomplete dominance, Bb brown, so all your kids will be brunettes,” Mandy said, “Oh brother.”

• Co-dominance, where both alleles are visible in the phenotype. For example, a flower with red and white spots, resulting from crossing a (homozygous) red flower with a (homozygous) white flower. Another example, the AB blood type (heterozygous), which has both A and B antigens, as mentioned
• Complementation, where a wild type phenotype is restored, from the crossing of two recessive mutations located on different genes, thus “complement” each other. Wild type is the phenotype as it occurs in nature. For example, a flower can be either white or red, the red color requiring a set of two working proteins, “A” and “B”. The dominant alleles coding for a working protein are “A” and “B”, and the recessive allele coding for a malfunctional protein are “a” and “b” respectively. Two different recessive flowers are crossed, including therefore “aa” and “bb”. Each flower however, does not have only one genotype, but two genotypes, meaning the genotypes should be propounded to “aaBB” and “AAbb”, noting that the flower is still white because both proteins are not present (as required for expression of the red color). Crossing over, the result will be “AaAaBbBb”, providing both the “A” and “B” protein required for the red color. In contrast, being located on the same gene, means being of the same allele, meaning “aa” and “aa”, or “bb” and “bb”. Again, the genotypes should be propounded to “aaBB” and “aaBB”, or “AAbb” and “AAbb”. Crossing over, the result will be “aaaaBBBB” or “AAAAbbbb” neither of which contain both “A” and “B”. This is why if two recessive mutations are crossed over, if a wild type is shown, it means the mutations are located in different genes on the two individuals

“I’m the wild type… a real bad ass girl!” Mandy giggled.

“You’re so not, Mandy,” Jamie replied, “I think you like to pretend to be bad ass on the outside, but on the inside, you’re a secret princess lol.”

“”Shut up, you do it too,” Mandy replied, “I mean, oohh I’m such a bad ass I’m Jamie, each force has an equal and opposite force, that’s soooo hardcore LOL!”

• Epistasis, where the expression of one gene depends on another gene. An epistatic gene is a gene, which suppress the effect of another gene. Evidently, whether the latter gene is expressed depends on whether the epistatic gene is present. For example, the gene causing albinism hides the gene controlling hair color, meaning the albinism gene is epistatic. A modifier gene is a gene, which doesn’t suppress, but alters the phenotype. For example, although one gene controls the eye color directly, modifier genes determine the final phenotype of the eye color
• Gene interaction is the collaboration of several genes to produce a single phenotype
• Polygenic inheritance, where genes have the same phenotype, such that the effects of each allele is additive. An example is skin color

Penetrance is the percentage of individuals carrying a gene, who also express its phenotype. Expressivity describes how the phenotype is expressed.

As time went by, the strong feelings that Jamie and Mandy had for another, although still there, had moved to the back of their minds . Rather, they were preoccupied with the truly important matters of Kingdom culture , such as generosity with time, money and resource, self-sacrificial nature, servant leadership, equal treatment of all within the church from the wealthy to the homeless.

As a result, rather than focusing on finding the ‘right’ person , they had inadvertently transformed themselves becoming more and more like that ‘right’ person .

Although Jamie and Mandy had given up all hope of being together, God was preparing the perfect timing.

Upon completion of her Engineering degree, Mandy gained employment in a hedge fund, before windfall gains permitted her to exit and start her own hedge fund, Mandy Cap, with a startup of \$500m.

Upon completion of his Law degree, Jamie went on to study medicine, inspired by Sophie’s tact in dealing with people, and became a pediatrician.

Sophie herself had her own lessons she needed to learn: that the “God” thing isn’t necessarily the “good” thing, and that in her self nature, although everybody thought of her as a good little princess, she had a deep dark chest of Sin that needed to be dealt with; the greatest of all which included pride. But that required her own adventure, that Jamie was not to be a part of.

Note that recessive traits are only expressed when homozygous (“aa”). Even if an individual doesn’t show the phenotype for a recessive allele, that allele can therefore still be present. Therefore, it is difficult for a recessive trait to become extinct, because it needs not only be deleted in its homozygous form (“aa”), but also its heterozygous form (“Aa”). Recessive traits are also less likely to become extinct, because there can be advantages, for example, sickle cell anemia. Sickle cell anemia shortens life expectancy in homozygotes, the heterozygote form confers resistance to malaria.

However, inbreeding is undesirable, as it increases the likelihood alleles are identical, thereby increasing likelihood of homozygosity, including homozygous recessive disease.

As stated , the twenty third pair of chromosomes is the sex chromosome, XX for females, and XY for males. The “Y” chromosome of the male is truncated. Because males lack two copies of either sex chromosome [as females have], males are subject to far increased recessive diseases at the X chromosome, known as X-linked recessive diseases. [Remembering expression of recessive alleles only occur when both alleles are recessive, and since there only is one allele, the chances dramatically increase.] In the Punnett square drawn for the sex chromosome, the male Y gamete has no alleles. This means the male will directly express what is received from mom, whether dominant or recessive. For example, the Punnett square for the X-linked recessive disease color blindness is:

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Notice that the male gamete “Y” has no alleles, meaning that colorblindness from the mother, is received directly. Another example of an x-linked recessive disease is hemophilia. Note that in the phenotype $XX^c$, colorblindness does not show as it is a recessive trait.

Although the female has a “XX” chromosome, an analogous effect can be caused when one of the X chromosomes in the female condenses, known as Barr body. The condensing of the X chromosome causes it not to express the majority of its genes. The choice for offspring, which X chromosome condenses, is random. Thus, half of the cells express their maternal X, and half express their maternal X. This is why calico cats are female, because the X chromosome determines fur color, and since males have only have one “X” chromosome, he cannot be multicolored. Females turn out to be patched in color, because half of the cells are one color, and the other half are another color.

Holandric traits are genes found only on the Y chromosome [and not the X chromosome], and are very few. In fact, genes on the Y chromosome are very few compared with the X chromosome. An example of a holandric trait are hairy ears.

Jamie developed a medical imaging technology, which required commercialization. Amongst 5,000 VC’s to which his business plan was sent to, responses came from only 600, and only 4 proceeded to due diligence, which included Mandy Capital.

Even as Jamie readied himself to present to Mandy Capital, he had no idea of Mandy’s involvement, because although owner, Mandy had stepped back from day to day management, to the role of Chairperson.

 Learning activity What are the concepts of Mendel?

2 Meiosis and genetic variability

Meiosis (different from mitosis, covered ) is a special type of cell division only available to germ cells, involving two divisions, meiosis I and meiosis II. Like mitosis, meiosis involves interphase, before heading into prophase, metaphase, anaphase and telophase. Since there are two divisions, PMAT is labeled:

• Meiosis I, which is where a regular diploid human cell (46 chromosome) is split into two haploid cells (23 chromosomes), known as reduction division. Thus, there is a reduction in the ploidy number, which is the number of sets of chromosomes in a cell, from 46 to 23. It includes:
• Prophase I, which best distinguishes meiosis from mitosis. Prophase I involves synapsis and crossover. Synapsis is the pairing of two homologous chromosomes [themselves pairs], one a paternal pair and one a maternal pair. The paired pairs are called tetrads, with a total of two chromosomes, or four chromatids. This permits crossover, which is the exchange of corresponding regions of chromatids between homologs, thereby resulting in recombinant chromosomes. Prophase I is the longest phase of meiosis
• Metaphase I, where like metaphase in mitosis, the chromosomes (here, paired homologs, or tetrads) line along the equatorial plane
• Anaphase I, where like anaphase in mitosis, there is pulling apart. However, here, the chiasmata (cf. sister chromatids) are pulled apart. Chiasmata are the points where crossing over occurs, and the only point where the tetrad paired pairs are last attached. Thus, the centromeres are not separated creating chromatids (half butterflies), but rather, two separate homologs (two full butterflies). Centromere is the part of a chromosome that links sister chromatids
• Telophase I, where like telophase in mitosis, the nuclear membrane is reformed
• Meiosis II, which is identical to mitosis, though its input [and therefore output] are different. Whereas a diploid cell is input into mitosis to produce two daughter diploid cells, a haploid cell is input into meiosis II to produce two daughter haploid cells. Like meiosis I, meiosis II is split into prophase II, metaphase II, anaphase II and telophase II

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In males, in spermatogenesis (discussed ), spermatogonia, a type of germ cell is arrested in mitosis until puberty begins. When puberty begins, spermatogonia, that is diploid (46 chromosomes), undergoes mitosis (synthesis) to form a primary spermatocyte, also diploid. The primary spermatocyte then undergoes meiosis 1 to produce two secondary spermatocytes, which are haploid (23 chromosomes). These two secondary spermatocytes eventually mature into four male gametes.

As Jamie began pitching, Mandy recognized him by his voice. However, he still failed to recognize her, as she had dyed her hair in blonde, though their eyes met.

“Dr. Swift, is it?” Mandy asked, once Jamie finished.

“我是你同時代的人之一? 你還記得我Mandy Marie Walker嗎?” Mandy replied in Cantonese, transliterated to meaning “I was one of your contemporaries? Do you remember me?”

“I can’t believe you actually learnt Cantonese, that really freaked me out LOL,” Jamie commented.

“Oh,” Jamie replied, thinking- well, stuck in thought. He didn’t know what to think, because his fear failed to subside, but rather, conglomerated with surreality.

“Can we recess whilst I have a word with Dr. Shum?”

In females, in oogenesis (discussed ), oogonium, a type of germ cell that is diploid (46 chromosomes), undergoes mitosis (synthesis) to form a primary oocyte, also diploid. Females are born with about 400,000 oocytes remaining by puberty. In contrast to males, primary oocytes are arrested in prophase I (a later stage) until puberty begins. At puberty, a monthly cycle occurs where one primary oocyte will undergo meiosis I to form a secondary oocyte. Oogenesis meiosis is distinct however, in that at each division, rather than producing two oocytes, an oocyte will be released with a polar body. Polar bodies are produced so extra cytoplasm is conserved for the egg (taken from the other 3 polar bodies). Therefore, in contrast to spermatogenesis which produced four male gametes, oogenesis only produces one female gamete.

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However, the secondary oocyte is arrested after meiosis I, and does not undergo meiosis II into an ovum, until after fertilization. Fertilization unites two gametes, namely sperm and ovum, to form a zygote. Zygote undergoes cleavage, which is cell division leading to increased numbers of cells, but constant mass, the compact mass now known as morula. Morula undergoes blastulation, which is the formation of a fluid-filled cavity known as the blastocoel, to form blastocyst. Blastocyst undergoes invagination (folds inward), known as gastrulation, to form gastrula. The opening where the invagination occurred is known as the blastopore, and develops into the mouth and anus. In protostome (from Greek “proto” meaning “first”, and “stoma” meaning “mouth”), the blastopore becomes the animal’s mouth (only). In deuterostome (from Greek “deutero” meaning “second”), the blastopore becomes the animal’s anus, and another hole is made for the mouth. A mnemonic to memorize this is that humans are deuterostomes, and were high enough on the evolutionary chain, not to eat through their anus, so developed another hole for their mouth. Gastrulation also forms a three-layered structure, including ectoderm, mesoderm and endoderm. Nail, skin, and nervous system are derived from ectoderm. Bone and muscle is derived from the mesoderm. GI tract [including glands which open into the digestive tract, such as liver and pancreas] and respiratory tract are derived from endoderm.

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Chapter 14: Genetics - Biology - Pre-med science - MR. SHUM'S CLASSROOM